Add a ground wire from each tail housing. Use one bolt to the housing and bolt the wire to the frame. You are experiencing a ground fault and the lights tend to work in strange ways with bad grounds. Just the wires for now and see if that helps. Then look at the stop switch. Power on one side and lights on the other. Pull the brake pedal down and see if you get brake lights. Then try the running/park lights. Then try the turn signals. One step at a time. A beer or two might help if you are so inclined. If it all works, then hide the ground wires you put on and consider what rust can do to a simple electric light system. Let us know what you find.
