[dsraven]

I usually find when you get weird outcomes like that it turns out to be a bad ground somewhere. start at the battery negative and run through all the ground connections for each part. each electrical part needs to have a dedicated ground to nice shiny bare metal frame ground. don't rely on body parts to supply a good ground. I find that when one circuit does something weird in another circuit it is because the ground isn't found in one of those circuits so it is finding a ground by running through another electrical componet and using that component for a ground. I usually ground all the lights out back to the actual truck frame, the lights out front to the actual frame and then the stuff in the middle thats not in the cab-also directly to the frame. the cab gets a seperate large ground wire from the engine or the frame to the main structural part of the cab. then the stuff iside the cab gets grounded to the main structural part of the cab as well. bare shiny metal, star washers and anticorrsion on the ground connections.