A frame to the body (box) ground is needed. I think if there is one it is usually near the back of the frame closer to the taillights on the drivers side. I guess I would try to clean them up. Pull the taillight housings and clean up the body where the screws hold the housing on. Go right to bare metal. Use star washers. Check all the firewall wire connectors too. Use a wire toothbrush and clean them up good. Spray them with WD-40 and blow them dry with compressed air. Then coat them with a little dielectric grease. Hope this helps.