headlghts

I think you hit on a dandy clue with your observation of low voltage if that was fuse D, a 40 amp under the hood. That fuse feeds the low beam head lamp relay and the door lock relays in the fuse box inside the car. To verify that is a valid clue, run a temporary jumper wire from that fuse to the big terminal on the fuse box that has the positive cable bolted to it, or to the positive remote jump start terminal. If the lights and locks work, there has to be a loose or corroded connection inside the fuse box.