I have a 1986 that has given me fits over the years. My years has had problems with the wiring harness that passes through the hinges. i pulled out all their wire and made my own harness.

Your problem is not the wires since your key lock is not siding the the pin attached to the locking motor back and forth. The pin has either become corroded or the linkage or the key cylinder is jammed. Its back to a pin to tight in the door button latching mechanism. I ground the pin down a little smaller to give it more clearance to slide when it gets rusted.

The key cylinder has to drag the motor linkage and push the pin in a hole, under the latch button so, you can open the latch, thats holding the door shut. Check to make sure the latch motor it self, is mounted tight. Another misalignment problem. These screws can work loose after repeated shutting/slaming of the tailgate. The support lifts may be getting weak and lets it drop hard. Sometimes hard enough to pop my third brake cover off. I got new one with a lifetime warranty at Kragens for about $20.

Good luck its a good project for the 4th of July or even later!