Dehodg,

Your symptoms are indicatitive of an open ground.

Check the ground wire on the trailer. Make sure it is making a good electrical contact to the frame if it uses the trailer frame as a ground.

Then check to make sure the ground on the vehicle side is sound.

When your trailer is hooked up to the hitch check to make sure you have an electrical ground path along the trailer wiring. If the lights work better when the trailer hitch is on the ball then you have a grounding (or return path) problem.

Good Luck....