The other day while wiring a bumper pull trailer for lights I noticed that my left brake lights and left turn signal would not come on. To check the wiring on the trailer I hooked it up to another vehicle and everything worked fine. This is for a 4 pole hookup. To me this says that I have a problem with the wiring on my truck for the 4 pole trailer hookup. What do I need to do to fix this. I would really like to avoid messing with a dealership. It is a 2000 F250.

 

I would buy a debug plug at Walmart for $4 (the trailer ligh test plug with LEDs in it, used for debugging trailer light problems). Anyone with a tow vehicle should have one of these for isolating trailer light problems. Using it is much easier than hooking up a trailer to test the lights.

Then check the bulbs in the truck. Then check fuses. Then look for physical damage to wiring that feeds those lights.

 

It's becoming more and more common to find corrosion inside the 7-way/4 way plugs at the rear of the truck..... open them up and take a look.....If you have a meter OR a simple test light check these wires. Use the ground pin as a ground and if no light, use a frame ground. If that works, the ground wire in the plug is broken.