I think this is the right track.

I had the rotting wiring harness (1982-1988, I think) on my 84, under the hood but also at the reverse light switch, the door lock switches, and a few other places.

I had a pair of wires rub through a quarter inch from the switch (mounted on the lock cylinder) and turning the key started my locks doing a fun little dance of lock, lock, unlock, lock, unlocked open me! haha too slow locked again... when I was trying to open my door. The wires were shorting intermittently, and thumping the door stopped it long enough to open it.

It could be entirely because of the tailgate lock switch and solenoid, so I would start by disconnecting it in the tailgate and inspecting the wiring at the hinges or at one of the body connectors in the driver-side door wiring channel. This basically runs the entire length of the interior from firewall to trunk along the indoors side of the car's edge.

My guess is: Driver's or other door lock is shorted to ground or from wire to wire somewhere - probably driver's lock, but maybe tailgate. That got the locking relay stuck on, which (along with age) burned it up. The fuse and relay socket may be damaged too. Find what circuit has the short, fix it (run a new wire, don't try to find the exact spot of shorting), then deal with the relay, socket, and fuse.

Good luck!