why does my third brake light turn on when I hook up my trailer and turn off when disconnected. Everything has been working fine forever until last night.

 

Are the brake lights on at the rear of the truck at the same time? It has to be a ground issue. Check another trailer also, see if it does the some thing, if it does then it is something on the truck.

Good luck, and keep us up to speed on this one.

Toyman

 

I think there may be a corrosion problem in the plug to the trailer.

Common sense tells me there have been no wiring changes in the trailer (I HOPE) so the one thing that can have a problem is the connection from truck to trailer - if it looks nasty in either end, it could make a cross-connect to the brake light circuit.

Weather and corrosion can do that, so "CLEAN 'EM UP!"

Worse comes to be, replace the bad connector and make sure you use dielectric grease or vaseline to protect against it in future.

*Brake Parts Cleaner Spray can be real helpful here

If this doesn't make an immediate result - something is going on in the trailer to send power to the brake light system.

Your third brake light should be on the same circuit as your other brake lights.

I need an observation - is it only THAT ONE LIGHT that comes on, or do all of the brake lights?

 

Does your 250 have factory trailer wiring, or are you using a T-connector?

Steve

 

If you are using electric brakes it is possible that you are feeding back through the brake circuit looking for a ground. . Greg