How to wire trailer lights, plugs and sockets

This article will explain how to wire trailer lights and plugs, and how to wire the supply socket on the vehicle.

Difficulty: Moderate

Instructions

Things You'll Need:

• Trailer plug and socket of your choice, chosen for your particular need.
• 7 core 3mm trailer cable
• Solder and Soldering Iron
• Wire Strippers
• Side-cutters
• Electrical tape
• Rubber Grommet (if needed)
• General hand tools
• Self-drilling screws
• Drill and #2 Phillips bit.
• Trailer lights required by law.
• "P" clips
• Blue ring terminals
• Ratchet crimping tool.

1. Find a suitable place to mount the trailer socket at the rear of the car, as close to the trailer hitch as possible. Take care around exhaust systems, and make sure it won't scrape; for e.g on a sloped driveway.
   
   Locate a grommet that seals a hole between the underbody of the vehicle and the interior. After checking on both sides that it is clear to use, cut a slot in the grommet and push the trailer cable through.

2. Now you have the cable in the car, locate which side of the car the main floor harness runs on, as this will have all the wires you need to tap into. Find the plug or loom that is before the taillights and using a test light, and the help of an assistant and the ignition OFF, ground the test light wire and have the assistant apply the foot brake. Probe each wire in the harness until the test lamp lights. Have them release the brakes. If the light goes out, this is the brake light wire. Note the colour of this wire.
   Repeat for: Tail lamps, Reverse lamps, Left and Right hand turn signals.

3. Once you have found all the wires, it is time to tap into them.
   
   The colours for standard trailer wiring are as follows
   
   Yellow - LH turn signal
   Green - RH turn signal
   Red - Brake lamps
   Brown - Tail lamps
   Black - Reverse lamps
   Blue - Auxiliary
   White - Ground
   
   Tap into each of the vehicles wires by stripping the insulation back, twisting the wires listed above for each light onto their respective wire, and soldering the connections. After they are all soldered, tape the connections and re-tape the loom if need be.
   
   Run the cable throughout the boot/trunk under trim panels so it does not get damaged by cargo etc.
   
   The white Ground wire will need to be attached to a clean metal section of the body. Crimp a ring terminal onto the wire and after sanding a small section back to bare metal, and using a Tek screw, secure the ring terminal to the body.
   
   Lightly coat the sanded area and terminal with multi-purpose grease or Vaseline to avoid corrosion or rust.

4. Working outside the vehicle, connect the wiring to the trailer socket, these normally have grub-screw terminals and are marked with which wire goes to which terminal.
   
   After connecting the wiring and assembling the socket, mount the socket to the vehicle.
   
   Some sockets have lugs to mount the socket, some have a seperate mounting bracket. After securing the socket to (usually) the underside of the rear bumper, make sure there is no slack in the cable underneath the car, but also make sure it isn't too taut either.

5. Using a test light, ground the wire and test each pin in the socket for power from the respective circuit when turned on. To test the earth, connect the clip to a known working positive supply on the vehicle and touch the probe to the ground terminal in the socket. The lamp should light indicating a good ground.

6. This step will start on wiring the trailer itself.
   
   Fit all lights to their intended position on the rear of the trailer, provided they can be wired while they are mounted.
   
   Hitch the trailer to the car.
   
   Attach the trailer plug to the 7 core trailer cable and connect it to the trailer socket on the car. Roughly measure how much slack you will need to leave between car and trailer and also allowing for normal and sharp cornering. Once measured and confirmed, using a P clip and Tek screw, attach the cable to the hitch rails on the trailer. Use a cable tie as well.
   
   Repeat this at 40cm intervals along the chassis rail of the trailer, minding the leaf spring mounts.
   
   Once the cable is at the rear of one corner of the trailer, attach the white ground wire to the trailer body, by sanding back any paint, crimping on a ring terminal and fixing it to the sanded area with a Tek screw.
   
   Coat the sanded area and terminal with grease or Vaseline to prevent corrosion or rust. Terminate the rest of the wires into a weatherproof terminal block, and run wire groups, in split tubing, to each light.
   
   For lights on the tailgate (number plate lamps/reverse lamps etc.), run a seperate ground wire using a ring terminal and fixing it to the same ground connection that was made previously. Run this along with any other wires to the tailgate, leaving enough slack at the base of the tailgate to avoid wire damage.
   
   Make sure all wires and cables underneath the trailer are cable-tied and/or held with P clips to the body securely.

7. Once everything is finished, connect the trailer plug to the vehicle socket and begin testing.
   
   In some cases, the indicators may flash faster due to the extra lamp on each circuit. Fitting of a larger capacity, direct replacement flasher unit or the use of LED lamps on the trailer will rectify this.
   
   You should now be finished and your trailer ready to haul.
   
   Hope this was of great help to you.
   
   -Ben

Tips & Warnings

• When crimping ring terminals on, it may be a good idea to solder the connection too, to avoid corrosion.
• I am not responsible for the possible damage caused to your vehicle's electrical system and/or wiring due to inexperience or bad work ethic or accident. You do this at your own risk.
• Don't come crying to me if you damage your vehicle, you were the one who came looking for information in the first place. Thankyou -Ben