My money is on a bad ground.

My '89 developed tailgate electrical issues about a year ago. I found the hinge harnesses were OK, HOWEVER: the black ground wire that ties the body side of hinge to the tailgate side was torn. Same wire was torn on left and right side of car. You can see those ground straps from outside of car with tailgate shut, just step up on bumper or get up somehow. Look down into hinge's groove and you can see the black wire.



I screwed a ground wire into the body frame, near where the tailgate struts are anchored (both sides of car). I ran the wires down into the holes in the sides of tailgate and fished them out at the bottom. From there the wires go towards the center where the 'factory' ground screws are. That fixed it.

Wife's car developed a similar ailment a couple months ago. Same routine over again. Problem solved.

When something electrical doesn't work properly when asked, and something else works instead (as in your wiper working when not asked), it's usually a bad ground. The thing with the bad ground is grounding via the one that works at the wrong time. It's working because it's carrying the other thing's current to ground.
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Sven: '89 245 NA, 951 ECU, expanded air dam, forward belly pan reaches oem belly pan, airbox heater upgraded, E-fan, 205/65-15 at 50 psi, IPD sways, no a/c-p/s belt, E-Codes, amber front corner reflectors, aero front face, quad horns, tach, small clock.