Sorry for another one but trailer light dont work right. If the running lights are on the brakes dont work. If the running lights are off the brake lights work fine. I cant drive at night this is a huge issue for an up and coming transporter. Its not the trailer lights ive eliminated that with my plug checker. Is it the rely? Blown fuse? Or common ford electrical mess up?

 

It sounds very much like a ground issue.
When to much current is drawn, the ground resistance drops to much voltage across the resistance due to the extra current flow of both light circuits..
Your tester won't tell that.
Good luck.

 

yes, ground is bad. If your connected to the ball, it should help ground. Clean the rust a off the ground wire bolt and make sure everything is clean. You can also use a jumper cables to ground the bumper to the trailer hitch as a test.