Improper grounding will always be an issue. If you know there is supposed to be a ground strap from cab to engine and there is not one then add one. Maybe there is one and you just can't see it, (maybe under the cab?). The lights are grounded to to the body, but if the body is not grounded to the engine then the only grounding will be between the cab and fram via the mounting bolts, and they are probably corroded. I would go to your local auto parts and buy a ground strap and install it. It certainly won't hurt.

52 Chevy 3100 5 Window, LS Swap, 4spd Auto, Vintage Air AC / Heat, CPP Power Front Disc Brake Conversion with bearing upgrade, Power Steering Conversion, LED Tail Lights