Wiring a trailer light board is a straightforward job once you know the UK 7-pin colour codes. Whether you are fitting a new trailer lighting board or rewiring an existing one, this guide walks you through the process step by step.
What you need
Before you start, gather the following:
• A trailer lighting board (7-pin or 13-pin, matched to your tow vehicle)
• 7-pin trailer cable (the correct length for your trailer)
• A wire stripper
• A screwdriver
• Electrical tape or heat-shrink tubing
• A trailer light tester (optional but recommended)
You can find all trailer lights and wiring at TrailerTek.
UK 7-pin trailer wiring colour codes
UK trailers use a 7-pin connector known as the 12N (12 Nominal). Each pin has a fixed function and wire colour. Mixing these up is the most common cause of trailer lighting faults.
|
Pin |
Wire colour |
Function |
|
1 |
Yellow |
Left indicator |
|
2 |
Blue |
Rear fog light |
|
3 |
White |
Earth (ground) |
|
4 |
Green |
Right indicator |
|
5 |
Brown |
Right tail / number plate light |
|
6 |
Red |
Brake lights |
|
7 |
Black |
Left tail / number plate light |
Note: pin numbering is visible on the plug and socket casing. Always work from the pin numbers rather than relying on wire colour alone, as some aftermarket cables use non-standard colours.
Step 1 — Prepare the cable
Run the cable from the front of the trailer to the rear, securing it to the trailer frame with cable clips to prevent it dragging. Leave enough slack at each end for the connections. Strip approximately 10mm of insulation from each wire at both ends.
Step 2 — Connect the plug
At the front end, connect the wires to the 7-pin trailer plug according to the colour code table above. Each pin inside the plug is numbered. Tighten the terminal screws firmly — a loose connection is the most common cause of intermittent lighting faults.
Once connected, fit the cable clamp inside the plug body to take the strain off the terminal connections, and snap the cover into place.
Step 3 — Wire the board
At the rear end, connect the wires to the trailer lighting board. Most boards have clearly labelled terminals or colour-coded connectors matching the 12N standard. Connect each wire to its corresponding terminal and secure the cable clamp on the board.
If your board has a number plate light, this is typically wired in parallel with pin 5 (right tail) or pin 7 (left tail) rather than having its own dedicated pin. Check the board's instructions if you are unsure.
Step 4 — Test before you go
Plug the trailer into your tow vehicle and ask someone to help you check each function in turn:
• Left indicator flashing
• Right indicator flashing
• Brake lights on when pedal is pressed
• Tail lights on with sidelights
• Rear fog light on when switch is activated
• Number plate light illuminated with sidelights
If any function fails to work, re-check the corresponding pin connection at both the plug and the board. A trailer light tester lets you do this without a second person.
Common trailer lighting faults and how to fix them
If your lights are not working correctly after wiring, work through the following checks before assuming the board or plug is faulty.
Indicators not flashing or flashing too fast. First check pin 1 (left) and pin 4 (right) at both the plug and the board. A connection that is loose or corroded will cause the indicator to fail entirely. If the indicator flashes but at the wrong speed, and you have fitted LED lights, this is almost always a flasher relay issue — see the LED section below.
Brake lights not working. Check pin 6 (red wire) at both ends. Brake light faults are often caused by the wire being connected to the wrong pin or a loose terminal screw. Also check the earth connection at pin 3 — a poor earth is one of the most common causes of brake lights failing to illuminate.
Rear fog light not activating. Pin 2 (blue wire) only activates when the vehicle's rear fog light switch is turned on. If the fog light does not work, confirm the vehicle switch is on before checking the wiring. Some older tow vehicles do not pass a fog signal to the trailer socket — check your vehicle handbook if in doubt.
Number plate light not working. The number plate light is wired in parallel with the tail lights rather than having its own pin. If the tail lights work but the number plate light does not, the fault is in the board itself or the connection between the tail light circuit and the number plate lamp. Check the board terminals and the lamp fitting.
All lights working but intermittently cutting out. This is almost always an earth fault. Re-check pin 3 (white wire) at both ends and ensure the board's earth is making good contact with the trailer frame if it uses a chassis earth. Corrosion in the plug body is another common culprit — clean the pin contacts with electrical contact cleaner and apply a small amount of dielectric grease to prevent future corrosion.
Legal requirements
Under UK law, all trailers used on public roads must display working brake lights, rear lights, indicators and a rear fog light. The number plate must also be illuminated when driving in the dark. A trailer with a faulty lighting board is a roadworthiness offence and may also invalidate your insurance. Check all lights before every journey, particularly if the trailer has been in storage.
Upgrading to LED trailer lights
If you are replacing a standard bulb board, it is worth considering LED trailer lights. LED boards draw significantly less current, last longer, and are less prone to vibration failure than filament bulbs. Wiring is identical to a standard board.
One point to be aware of: some older tow vehicles use a load-sensitive flasher relay that depends on the higher current draw of filament bulbs. Fitting LEDs can cause the indicators to flash too quickly. If this happens, a load resistor or an LED-compatible flasher relay will resolve it.
Shop trailer lighting at TrailerTek
TrailerTek stocks a wide range of trailer lights at TrailerTek, including lighting boards, LED conversion kits, plugs, connectors and wiring. Call us on 01962 774 988 if you need help choosing the right parts for your trailer.
