Well its kinda hard to explain so im going to try my best. Anyway i have a flatbed bumperpull that i tow with my 2000 7.3. The other day I hook it up and turn on the running lights and get nothing which is odd because they had worked a few weeks before that. It was raining that day and I thought that may be the problem well hook it up the other day been super dry and still no lights so took a test light and tested the wire behind the plug on the truck and no power in wire. So I try hardwiring the license plate light to the plug wire and the lights on the trailer came on then i tried the blinker and they went out again then started flickering. Its like the wire was backfeeding to the truck or the truck was trying to power the trailer again but I have no Idea. Im no good with wires. My electric brake system freaks out in the rain too. It says that it has a trailer hooked up to it so I think water is getting into the plug. Think I might have fried the electric brake controller again too because when I hooked the trailer up the controller said that the trailer was on the truck and the numbers would flash up but not apply the brakes on the trailer. Could also be that the ball was rusty and not grounding but idk. Hope someone can help.
Thanks for the help

 

There is a separate fuse for the trailer running lights, that could be blown. What caused it to blow could be where the issue is...by the "backfeeding" issue, I would inspect the trailer wiring to make sure nothing is smashed or cut, shorting.

And the ball shouldn't really be the ground, the white wire should be the path of the lighting, which in most cases is connected to the trailer frame anyway, but what I'm saying is you shouldn't even have to have the ball connected in order to get the lights.