Troubleshooting Trailer Wiring Harness Where Brake Lights Only Work When Running Lights Are On.

Expert Reply:

I have a couple of troubleshooting steps you can run through, but this may just be a common wiring malfunction. I would start by using a circuit tester like the # PTW2993 and testing the pins on the 7 way to see if power is being supplied to all of the pins; if you're getting power on all 7 pins then the malfunction will be on the trailer side and not the vehicle wiring.

If the issue is on the vehicle, then you'll want to start with checking the ground wire to make sure it's not loose, and is firmly connected to a paint and rust free spot on the frame. You'll want to then check the harness again if you've made any repairs to see if it is supplying power to all of the pins properly.

If the 7 way is providing power completely then you'll want to check the trailer wiring for any damage, specifically on the brake light wires; replace any damaged wiring as you go.

I've also linked some troubleshooting FAQ articles that we've done that you can check out and use as a guide when looking for pin diagrams on your 7 way and trailer plugs. If the above mentioned steps don't resolve anything, just let me know and I will give you the next couple of things to check out.