In life, there are some things that are just not cool. Catching fire due to a lame-ass electrical job is one of those things, so make sure you understand what you are doing before randomly cutting and “twist and taping” wires.

Difficulty: 5/10

Duration: 0,5h to 2h

Tools/supplies:

• Digital Multimeter
• Splice connectors

 

First of all you should find out if the rear lights are linked to the CAN-BUS (the car’s computer) or not. The easiest way to tell is to check the car’s user manual and see if the car is able display any kind of “burnt bulb” error for the rear lights. If it does, then that is a real tell sign that the rear lights are CAN-BUS linked.

Why is this important? Because the CAN-BUS works by constantly measuring the load for each single light bulb. Any significant change in load, such as the light bulb burning (yielding zero load) or adding an extra bulb (doubling the load) will start prompting “burnt bulb” errors or even making the lighting module malfunction (in the case of overloading it). Therefore, it is not possible to directly connect the tow-bar electrical system to the tail light bulb wires.

For more CAN-BUS info read this article: Wikipedia CAN-BUS

For (older) cars that don’t have CAN-BUS tail lights, depending on the load rating of the car’s electrical system, you could get away with directly connecting the trailers to the car’s tail light wires.

Otherwise, and/or you want to be 100% safe, a relay must be used. In the simplest terms, a relay acts as an auto switch, keeping the trailer’s lights on when the car lights are on, and shutting them off when the car lights are not on.

A simple schematics of a relay used for trailer lights (needed for each individual light):

 

 

There are tons of “trailer relays” available, and while they work great convenience-wise, they have the disadvantage of having only one fuse for all the lights. If I were to use relays, I’d build my own relay and fuse each light separately.

Judging the thickness of the wires, and fuse rating, I decided to directly link the trailer lights to the stop lights for my Mondeo.

No matter which route you take you need to splice into the tail light wire loom. That is accomplished by measuring the voltage for each wire with the multimeter.

Begin by searching for the position light, and establishing the ground. With only the position lights on, start probing each bulb wire, holding one end of the multimeter on a fixed (metalic) location, and moving the other one through the rest of the available position, until there is a reading on the multimeter (12.xx V). Should it display -12.xx, just reverse the multimeter ends. Now, write down the position of the positive multimeter end (it’s the wire  for the position), and the negative one (it’s the ground). By holding the negative multimeter on the ground, start turning on the other light functions (turn signals, fog, reverse, etc), one at a time, checking which new wire got voltage.

If you are lucky, you could find a wire diagram online. For the Mk3 Mondeo, I’ve already done the work for you:

 

For the trailer plug, the standard coloring for a 7 pin plug is below, however just to stay safe, make sure you double check.

After identifying all the wire schematics, is time to splice into the wires.

DO NOT TWIST AND TAPE!

The best way to splice into wires is to remove some insulation, solder and cover in heatshrink tube. The second best way is to remove some insulation and use a butt crimp connector. The third best way is to use a splice connector. The latter is the quickest, however the bulkiest and with the highest risk of coming loose.

Plurihack tip: secure the splice connectors with thin tightly fit plastic ties.

 

Before mounting all the covers, make sure to check if all lights are operating properly.

Now you’re done! Enjoy your trailer!