Originally Posted by TBApollo
I removed the ground, replaced the screw and sanded the contact point to bare metal and sanded the place that contacts the trailer ball.
I had similar problems with my trailer. It turned out to be an insufficient ground.
(I don't understand your sentence above regarding the trailer ball. Are you using the trailer ball as part of your ground circuit?)
Neosec's diagram is correct and will work IF you have a good ground connection between the vehicle and the trailer. If you don't have a good ground wire connected to your vehicle that mates with the white wire in the trailer harness that is securely bolted/screwed to the trailer frame, try installing one. Or try using a set of jumper cables to connect the tow vehicle frame to the boat trailer as a test to see if a better ground will get this working.
On my trailer I had a ground wire connected correctly, but it turned out that it was too small of a wire to do the job. Once I added a second, larger ground wire, all my weird trailer light issues went away.