Sounds like a grounding problem to me as well.

Replace the multi flasher and double check that harness you unplugged. One thing that bothers me is you say 1 light is brighter than the other, you could have the bulb in backwards. Turn on the head lights on turn on the blinker, go back and see if the light blinks brighter OVER the tail lights, if not you have the bulb backwards.

Personally, I think your going to find your trailer light socket the culporate. You can pull your socket out and disconnect it on the back side. If your problmes goes away then that is the issue. You have to go to a dealer to get a new trailer socket.

 

As it turns out, all of the problems came from a bad headlight switch. I finally gave up and took the truck to the local garage where they diagnosed it in just a few minutes. $127 for the switch and a little labor, and I'm good-to-go again. thanks for all of the help.