1976 Chevy Truck Lighting Problem

All ground wires should be black or copper braid. On older vehicles like this alot of grounds have been disconnected. You shold have a ground from battery to body. Battery to engine, engine to body at the very least. Check these grounds first. There may be a ground wire going to the base of the steering column that will go to the dash as well. I'd be one of the body grounds are off. Body to engine and body to battery should be 10 guage wire. It could be something other than a ground as well especially if it's had an alarm system or radio installed incorrectly.