I have an 01 Screw 4x4 that I bought back in February and I am desperately trying to go fishing (freshwater), but for this odd trailer light issue. For background info this is a very straight forward trailer for a 14 ft john boat. I have to use an adapter because the trailer plug could not reach the truck plugin and was a flat 4. First, I have tried 2 adapters so I don't think its an adapter issue (unless I have the wrong type of adapter). Second, I hooked it up to my dads grand marquis which doesn't use an adapter and everything worked fine. Also I have checked the trailer fuse in the panel under the hood and under the steering wheel and they are good (unless there is a 3rd fuse for it I don't know about). Now for the problem.

With the headlights off nothing will work as far as trailer lights, no running lights, no brake, no blinkers....

With the headlights on, the trailer running lights will work and the blinker lights will work, but if you hit the brake it will cause the trailer lights to go out all together.

I don't know what could be causing this phenomenon. Thanks for any help.

 

Poor ground.

Make certain the white ground wire is securely attached to the truck's frame and, on the trailer, that is is attached to the trailer chassis.

Your symptoms are commonly seen when trying to use the ball and hitch for the ground which often gives weird results.