I am having a problem with my trailer lights and my truck. The lights on the trailer (have tested on two different trailers with the same result) will not work unless my truck's lights are turned on. This goes for running and brake lights. Anyone have any ideas or seen this before?

 

The trailer's parking lamps will not be on unless the truck's parking lamps are on, this is how it would work.

If you are saying that the brake lamps will not work on the trailer unless the truck's parking lamps are on, there is a problem.

Is this for the 4 pin or the 7 pin or both trailer tow adapters ?

When you meter the ground on the truck side of the trailer to a know good ground on the truck, what is the resistance ?

Is the trailer tow adapter on the truck clean ?

 

I'm sorry I meant to say the brake lights won't work unless the truck's lights are turned on. The truck has a 7 pin connection with a 7 to 4 adapter on it. This may be the problem. The 7 pin connector looks like it may be plugged into a 4 pin itself up and behind the bumper, so I will try playing around with it and see if that could be the problem.

 

I have a 89 f150 I put a tee in the wire harness with the 4 prong to hok up to my trailer only the brake light works, WHY? i have the same hook up on my brothers trk his works fine (its a dodge)