Troubleshooting Trailer Wiring that Stops Working When Headlights are Turned On

Question:

I have a 2012 vw Passat with a modulight hd 119147. I plug it correctly but when I turn the parking light on, I have no more brake and turn signal on the trailer and the light are like if am braking.

asked by: Martin

Expert Reply:

Try improving the connection of the main ground wire of the trailer wiring harness. Make sure it is attached to a clean metal surface. Or you can run all of the trailer wires back to the main ground. This is actually the ideal solution. But you can also check the current grounds for the lights to make sure they are making a solid connection to a clean metal surface.

What you described is the classic insufficient ground scenario so finding a way to improve the grounds will fix the problem.

expert reply by: Jameson C