I think you are right; its in the wiring most likely, and possibly an actuator that has filled with water, though I'd think that PTC "resettable fuse" would protect from that.

From my '91 WDM, there's the tailgate harness connector on the RIGHT side, under the headliner, below the hinge. You could disconnect there to eliminate the t-gate short circuit suspect. The headliner is easy to deal with if you see it is just stretched over a lip in the corner.

The actuator wires are blue and yellow; there's another connector supposedly, under the carpet, left of the tranny tunnel, just beneath the front console kickplate. I remember a number of connectors down there, so again, look for one with the blue and yellow wires. It should isolate the rear doors and the t-gate from the front door circuit.
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Art Benstein near Baltimore