Troubleshooting Brake Lights and Running Lights Not Working On Trailer at Same Time

Question:

Why if I put the car lights on trailer lights go on but if I put the car lights on the break lights will not work on the trailer but will work separately

asked by: Nick

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