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Take a piece of wire, attach it to the ground at the trailer tongue. Then try to find a spot on the tow vehicle to ground it to (bare spot on vehicle frame) If you get lights then the plug on the vehicle side does not have a good ground.
This is the way to trouble shoot the problem. It is my experience that the ball and hitch do not make a good ground. That ground wire inside the 4 pin plug is needed for lights to work and the vehicle side has to go to a good ground while the trailer side also has to be grounded to the trailer frame. The other things I can think of are the turn signal flasher which might need to be changed for a heavy duty one, or if the tow vehicle uses a computer for controlling the lights, you will need relays for left, right, and tail lights. Check the owner's manual. Those computers will cause really strange things to happen, so go to Radio Shack and buy three 12 volt relays and wire them in. I had to do that on my 1995 Crown Victoria. If your vehicle has a trailer package, the relays should already be there. I have a 1995 F350 that has relays even for the backup lights.
Last edited by LakeSuperiorGeezer; 11-12-2011 at 12:29 AM.