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Trailer lights stopped working, problem is with the car. How to troubleshoot?

Hi there I was in the process of replacing the lights on my box trailer in doing so I was playing around with the cables to figure out the correct wiring. In the process I think I somehow managed to blow a fuse or damage something in the car because the lights on no longer working I have tried connecting different trailers that I know work including my portable spare trailer light panel and none of them work.

I have checked all of the fuses in the car using a multimeter and none of them seem to be broken the cable itself is perfectly fine I can’t see any issues there plus it worked prior to me testing the trailer wiring.

Does anyone know what the problem might be and how I can troubleshoot it. Another clue is that the rear electrical Windows also stop working at the same time.

The car is a Volkswagen Jetta 2006 it did not come with the trailer lights installed that was done aftermarket.

PS I’m too cheap to take this old banger to the repair shop.


Do the tail, brake, ant turn signal lights work on the rear of the car. I would use a 12 volt test light rather than a multi meter but that may be just because I am old.

If the car lights work, the first thing I would check is continuity of ground from the ground wire on the trailer to your car frame. Yes, you would need anothe light for that. Then test for power at the appropiate pins of your trailer plug. Then go forward in the wiring until it connects to the car wiring,

If you don’t have working lights on the rear of the car, keep testing forward until you hit power or a fuse or the multifunction switch.

If you have a wiring diagram for your car, see what else is on the same circuit as your rear door windows.

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Yes, all the lights on the car work just fine, but I’m not able to measure any power coming out of the plug itself, from any of the pins, completely dead, which is why I was certain it was a fuse.

In terms of the trailer lights, they are working, I ended up using my car battery to test them manually.

I’ll see if I can follow the wiring that’s a good idea, and will see if I can source a digram.


Follow the trailer plug to the source… i.e. where was it added to the wiring harness. If they used these horrible, nasty, useless things - scotch locs - assume THAT is the problem;

Scotch Locs, in theory, cut thru the insulation and make the connection. Sometimes they cut thru the wire as well but they ALWAYS let moisture in so the wires corrode. The wires themselves may become the fuses and they burned up. OR the ground wire burned up. In any event, you need to follow the wires to their connection points.

Your car has separate lighting circuits for the brakes and turn signals. That is a 6 wire circuit. Chances are your trailer has the brake and turn signals lights combined on a 4 wire circuit. To make this transition, a small box is wired into the cars circuits and a separate 4 wire circuit to the trailer comes out of the box.

Just follow the trailers pigtail back to the box. You will need a new box. Last time i got one it was about $25 but that was over 20 years ago.