This thread was answered with the solution. If you have the same problem(lights will not work with the parking lights on, they will otherwise), take a long scrap piece of wire and hook it to the battery negative terminal, run it in the yard back to the trailer, and hook it directly to the ground for the light at the light itself, and see if that cures it on that side.

If it doesn't, then take the light apart and with the parking lights on, and the turnsignal to the light you are working on activated, jamb your temporary ground wire beside the light so it touches the brass part of the bulb. That should fix it. If it does, then you have a connection problem between the brass part of the bulb(bulb ground) to the ground wire of the light fixture. If running the temp ground wire directly to the ground wire of the fixture fixes it(the first test) then you know you have a ground problem between the fixture ground wire and the frame of the trailer, and or the trailer frame and the truck frame.

Don't rely on the ball to give you a ground, all trailer wiring plugs have a spot for a ground to pass through the plug itself.