an electrical nightmare , please help , 1972 dart

"Maybe." What have you done to check grounding and do you understand how? You HAVE to get access to the lamps and sockets involved. "Let's say" the headlight grounding is poor. The ground leg of both filaments in the headlamps are common, so if the lights are powered on to either hi or lo beam, the ground common will feed power back into the unused filament and appear with your voltage checks