[Myoldtruck]

Check and fix all grounding wires. Im going thru this with my 71 I just bought. Same issues as you. I added and fixed all grounds.

I used my Dremel to clean to the bare metal all my grounding straps. I added a battery cable from the engine block to the frame. I added a no.#6 gauge ground wire from the rear of the bed to the frame, using copper wire mounting lugs. Fixed grounding from cab to frame under the passenger side door.

You may have to clean the bulb connections in the rear where the bulb goes into. clean the brass and the two lead dots the bulb touches.

I have lights now.