Putting together a trailer lighting kit: a comprehensive guide for professionals

In this guide, you’ll learn how to put together a complete trailer lighting kit as a professional. You’ll be guided step by step through the key decisions, from the application and required light functions to the connection, cabling and final inspection.

In this article, you will read:

• what components make up a complete trailer lighting system;
• what decisions you need to make in advance;
• how to match lighting, connectors, wiring harnesses and wiring harnesses;
• whether individual components or a plug-and-play system are suitable;
• how to prevent common interference and fitting errors;
• What you need to check before delivery.

What is a complete trailer lighting kit?

A complete trailer lighting kit forms the electrical and mechanical link between the towing vehicle and all the light functions on the trailer. The system starts at the vehicle plug and runs via the main wiring harness, branch cables and connectors to the various lights.

• Connector: forms the connection to the towing vehicle.
• Wiring harness: distributes power, mass and signals throughout the trailer.
• Connectors and branch leads: connect the wiring harness to the individual lighting components.
• Rear lights: combine functions such as rear light, brake light and indicator.
• Auxiliary lighting: includes, amongst other things, marker lights, side marker lights and licence plate lights.
• Reflectors: ensure recognisability and visibility even without active lighting.

The quality of the system is determined not only by the individual components, but above all by their mutual compatibility and the way in which everything is connected, protected and fitted. A logically structured system reduces installation time, simplifies diagnostics and makes it easier to replicate the same configuration in series production.

Step 1: Determine the application

Don’t start with the light, but with the trailer and its intended use. The type of vehicle, its size, its application and the preferred mounting method determine which configuration is suitable. You should therefore establish the key parameters in advance.

• Are you looking to build a new system, replace an existing one, or expand an existing system?
• What are the trailer’s length, width and body configuration?
• Which light functions are required for this model?
• Is the trailer used occasionally or intensively?
• Is the installation regularly exposed to water, dirt or road salt?
• Is it for a single vehicle or a repeatable configuration for series production?

When replacing parts, also inspect the existing installation. Check the connector, cable lengths, connection points, connectors and the condition of the cabling. Existing wires may contain hidden cable breaks, corrosion or damaged insulation. Where several components are obsolete or have unclear connections, replacing the entire wiring harness is often more efficient and reliable than carrying out a partial repair.

Step 2: Choose the correct system voltage

The system voltage forms the basis of the entire installation. Standard light trailers are usually connected to a 12V on-board electrical system. A 24V system is mainly found in trucks, trailers and semi-trailers. Although many lights are suitable for both 12V and 24V, this does not automatically apply to all components of the installation.

System	Typical applications	Important focus
System12V	Typical application: Light trailers towed by passenger cars and commercial vehicles	Important consideration: Compatibility with the vehicle’s electronics and the chosen plug connection
System24V	Typical applications: Trailers, semi-trailers and other applications towed by trucks	Important point to note: Components, connections and cabling must be suitable for the entire 24V system
System12/24V components	Typical application: Suitable for use in both voltage environments	Important point to note: Also check the voltage compatibility of control boxes and other system components

This guide focuses primarily on 12V trailers. For each configuration, determine the voltage supplied by the towing vehicle and then check each electrical component for suitability. This will prevent malfunctions and unnecessary diagnostic work.

Step 3: Determine the required light functions

The type of lighting required on a trailer depends, amongst other things, on the vehicle category, dimensions, date of first registration and intended use. You should therefore first determine the required light functions and only then decide which lights are needed to fulfil these functions. The key components of a trailer lighting system are:

• Rear lights: these usually comprise the rear light, brake light and indicator. Depending on the model, a rear fog light, reverse light, reflector or licence plate light may also be integrated.
• Licence plate lights: can be fitted separately or integrated into a suitable rear light.
• Marker lights: make the trailer’s outline more clearly visible, particularly from the side.
• Side marker lights: indicate the outer width of the vehicle in applications for which they are intended.
• Rear fog light: makes the trailer more visible in conditions of severely restricted visibility.
• Reverse lights: indicate that the combination is reversing and, provided they are fitted correctly and approved, can illuminate the area behind the trailer.
• Reflectors: help make the trailer visible when the active lighting is off.

For the final configuration, always refer to the requirements applicable to the specific trailer. In our overview, you can read more about the regulations governing trailer lighting. The practical requirements for positioning and mounting are covered in more detail in ECE R48.

Step 4: Select the rear lights

Rear lights play a major role in the surface mounting of the set. Don’t just look at the design and available mounting space, but focus above all on the functions and connections. A model that integrates multiple functions can reduce the number of individual components and connections.

When making your selection, please check:

• which light functions are included in the luminaire;
• whether a left-hand, right-hand or universal version is required;
• what sizes and mounting points are available;
• whether a reflector or licence plate light is integrated;
• which cable outlet or connector is used;
• whether the light is suitable for the selected system voltage;
• or whether the necessary ECE functional approvals are in place.

For product selection, view the overview of trailer rear lights. Match the chosen connection directly to the wiring harness. This prevents the need to adjust connectors or make loose connections during mounting.

Step 5: Choose between 7-PIN and 13-PIN

The connector determines which electrical functions can be transmitted between the towing vehicle and the trailer. A 7-PIN connector supports the basic functions for standard trailer lighting. A 13-PIN connector offers additional connection options, for example for reversing lights and supplementary power supply.

Connection	Suitable for	Selection criteria
Connection7-PIN	Suitable for: Basiclight functions on simple trailers	Selection criterion: Suitablewhen only the functions of the 7-PIN connection are required
Connection13-PIN	Suitable for more complextrailer configurations	Selection criterion: Moreterminals available for additional functions and power supplies

Make this choice before specifying the wiring harness. Check the required pin configuration, the connection to the towing vehicle and the functions that actually need to be controlled. For the wiring diagram, please refer to the explanation of the 7-PIN connector or the 13-PIN connector.

Step 6: Specify the wiring harness and connectors

The wiring harness connects all the components and largely determines how quickly, neatly and reliably the kit can be fitted. A suitable wiring harness has the correct connector, sufficient length, branch connections in the right positions and connectors that match the chosen lights.

When specifying, please ensure you include at least the following:

• 7-PIN or 13-PIN connection on the vehicle side;
• length of the main cable from the drawbar;
• position and length of the branch circuits;
• number of connection points for rear lights and auxiliary lighting;
• lamp connector type or open wire end;
• necessary cable protection and strain relief;
• Route the cabling away from moving, sharp or hot parts.

When making a connection to the towing vehicle, ensure there is sufficient cable length to accommodate steering and articulation movements. The additional length required depends on the drawbar, the mounting position and the range of movement of the combination. Avoid both a cable that is too tight and excess cable that could drag along the ground.

For auxiliary lighting, pre-assembled branch cables or dedicated plug-in connectors can speed up mounting. Use these exclusively within a wiring system for which they are designed and fit them in accordance with the manufacturer’s instructions. The quality of the seal is determined by the entire connection, not just the connector itself. View the available 12V wiring harnesses for trailers, as well as connectors and adaptor plugs.

Individual components or a complete plug-and-play set?

A trailer lighting system can be built from individual components or as a pre-configured plug-&-play lighting system for the trailer. The right choice depends primarily on the level of customisation required, the production volume and the desired installation time.

Individual components	Plug-and-play system
Individual components: plentyof flexibility for unique vehicle configurations	Plug-&-play system: Components and connections are pre-matched
Individual components: Suitablefor occasional replacement or bespoke solutions	Plug-and-play system Suitablefor repeatable production
Individual components| Additional wiring and connections on site	Plug-&-play system: Shorterand more predictable installation time
Individual components: Need more control over compatibility?	Plug-and-play system: Reduced risk of mixed-up connections
Individual components: Flexible when every trailer is different	Plug-&-play systemConsistent design and quality for every vehicle

For vehicle builders who frequently produce the same configuration, a standardised plug-&-play set usually offers the greatest benefit. Fewer manual connections reduce the risk of errors and make mounting and final inspection more reproducible. For unique vehicles or specific installation scenarios, however, individual components can actually offer greater configuration flexibility.

CAN-bus and compatibility with the towing vehicle

LED lighting generally consumes less power than halogen lighting. Some towing vehicles use the electrical load to detect lamp failure or the presence of a trailer. If the measured load deviates from what the vehicle’s electronics expect, this can result in error messages, flashing lighting or malfunctions.

The term ‘canbus proof’ alone is therefore no guarantee that every combination will function without issues. Operation depends on the towing vehicle, the trailer module, the lighting used and the complete electrical configuration. It is therefore advisable to truck a new or standardised set using representative towing vehicles.

If the vehicle’s electronics detect insufficient load, a suitable control box or other vehicle-specific solution may be required. Such a module simulates an appropriate electrical load or adjusts the signals to match the LED system. Select the solution based on the vehicle, the plug connection and the functions causing problems. Take a look, for example, at the 13-PIN 12V LED control box.

Avoid common causes of failure

Interference often does not originate in the light itself, but somewhere in the electrical or mechanical assembly. The points below therefore deserve special attention during design and mounting.

• Poor mass: an unstable mass can affect several light functions simultaneously and cause unpredictable behaviour.
• Moisture and corrosion: inadequately sealed or incorrectly fitted connections increase contact resistance and can ultimately lead to failure.
• Incorrect pin configuration: functions may be swapped or fail completely if the connector and wiring harness are not connected according to the same scheme.
• Mismatched connectors: adaptors and manually modified connections introduce additional points of failure.
• Incorrect cable length: cables that are too short are subjected to tensile stress; cables that are too long may rub, kink or dangle loosely.
• Inadequate cable protection: sharp edges, moving parts and flying debris can damage the insulation and connections.
• Compatibility issues: combining LED lighting with vehicle electronics may require additional configuration or a control box.

Water resistance and durable mounting

When used in rain, mud or cleaning environments, lights and connections must be suitable for the expected conditions. An IP rating describes the protection of the tested housing against dust and water under specified truck conditions. IP68 relates to dust-tightness and immersion in accordance with the conditions specified by the manufacturer. IP69K relates to cleaning with water under high pressure and at high temperatures.

A high IP rating for the light does not automatically make the entire installation waterproof. Cable exits, connectors, seals and the method of installation must also be carried out correctly. Where possible, position connections away from direct exposure to water, prevent water from running along cables towards a connector , and follow the manufacturer’s installation instructions.

Pre-delivery inspection

A lighting system is only ready for use once its operation, mounting and connection have been systematically checked. Do not limit the final check to simply ensuring that all lights are working. Also check that the configuration has been set up correctly and remains reliable under real-world conditions.

• Are the rear lights, brake lights, indicators, rear fog lights, licence plate lights and any additional functions working correctly?
• Are the left and right functions connected correctly?
• Do the connector, pin configuration, wiring harness and lamp connectors match?
• Are the ground connections stable and free from unwanted contact resistance?
• Are the lights, reflectors and other features correctly fitted and visible?
• Are the connectors fully closed and the seals fitted correctly?
• Is the wiring harness protected against tensile stress, abrasion, sharp edges and moving parts?
• Will the installation continue to function reliably when the drawbar and cable are in motion?
• Is the system compatible with the towing vehicles for which the trailer is being built?
• Has the chosen configuration been clearly documented for maintenance or for the next unit?

By making this check an integral part of the handover, faults are identified earlier and the risk of recurring interference and repair work is reduced.

Frequently asked questions about trailer lighting kits

What’s included in a complete trailer lighting kit?

A complete set usually consists of a vehicle plug, wiring harness, connectors, rear lights and the necessary auxiliary lighting. Depending on the trailer, this may also include marker lights, side marker lights, licence plate lights, reflectors and a control box.

Do I need a 7-PIN or 13-PIN connector?

A 7-PIN connector is suitable for basic light functions. A 13-PIN connection offers connection options for additional functions, such as reversing lights and an extra power supply. The trailer’s functions and the towing vehicle’s connection determine which version is suitable.

What is the difference between 12V and 24V trailer lighting?

12V is commonly used for light trailers towed by passenger cars and commercial vehicles. 24V is mainly used for trailers and semi-trailers towed by trucks. Lighting and other electrical components must be suitable for the voltage of the entire system.

Is every LED trailer lighting unit canbus proof?

No. Furthermore, the designation ‘canbus proof’ does not mean that a light will work without any problems with every towing vehicle. Compatibility depends on the vehicle’s electronics, the trailer module, power consumption and the overall configuration. Sometimes a suitable control box or other solution is required.

When should I choose a plug-and-play lighting set?

A plug-&-play kit is particularly useful when installation time, a low risk of error and a reproducible setup are important. This applies, for example, to mass production. Individual components offer greater flexibility for unique trailers and non-standard installation scenarios.

Why do trailer lighting systems sometimes fail to work properly?

Many interferences are caused by a poor earth connection, moisture or corrosion in a connection, damaged cabling, incorrect pin configuration or incompatibility with the towing vehicle. You should therefore check the entire circuit, not just the light that is malfunctioning.

Putting together professional trailer lighting

A suitable trailer lighting kit starts with a clear system specification. When the application, voltage, light functions, connection and cable routing are defined in advance, the components can be selected specifically and mounting becomes more predictable.

TRALERT® supports vehicle builders, engineers, buyers and distributors in putting together complete trailer lighting solutions. View the overview of professional trailer lighting or contact us for assistance with a specific configuration.