Boy, this looks like non-fun.

Make sure you have a good ground at the motor. One of the 3 motor mount screws goes through a hole in motor with metal sleeve and contact area - should be clean. Screw should be clean also.

If still no go,
I'd pull the cover shell off the steering wheel area, look at the wiring to wiper switch there. Our '86 had a ground wire come loose on left side of wheel, everything run by the left-side stalk stopped working and also the horn quit. Dunno if there's another ground wire on right side. I'd look in both places. Maybe test the switch itself with a continuity tester or a multi-tester if you have one available.

I'm pretty sure the '87s were still in the era with lots of (now) bad wire harnesses. Usually shows up in engine wiring, but could be anywhere, I guess. If you find rotten insulation etc., you could bypass the existing wiring and build new. I definitely would look at the other items above first.