When you look at your lights you will see there is only 1 wire for each, the ground is at the light on the trailer. I am not if you have screws or rivits at each. If its screws back them out and rescrew them to remove any build up it they are rivets I would replace them. You have another main ground where the bundle comes up on the trailer check that ground too. you may have a worn wie if the wrong light comes on and it is shorted check all connections and redo the seal and tape.
Hope that helps.
Jim (SSG US Army Ret.) and Cheri (TSG Phx ANG Ret.) Mesa, AZ
2006 Dodge Ram 2500 HD Mega Cab Diesel | 2005 Honda Goldwing | 2006 35' Dune Chaser 5th Wheel