Originally Posted by carltonwebb

Whenever i have had that exact problem, it has been in the ground side of the plug. Either the trailer or the truck. I know you have tried 2 different trailers, but horse trailers are rarely used every day and will corrode the pins on the connectors as stated earlier. I worked on one locally for a lady from S. Carolina that had spider webs in it. Her trailer full of horses had pushed her all the way here. Get you a can of electrical contact cleaner and clean them really well and see if it does it. Spray it on the truck side connector too.

However, it could still be the controller as well.