I've several of these trucks, and like mentioned before, the headlight switch and floor dimmer switch are pretty weak. I would never perform a test with another old switch, only new ones, as they just aren't expensive.

Also, the bulkhead connectors can be problematic.

But the single best upgrade I've done to all of my trucks was to simply make a new, separate wiring harness to all of the lights that becomes a dedicated ground circuit, straight to the battery! The difference this makes is amazing! While you are at it, make sure all of the ground straps are still there, attached, and clean.