My money is on either of two possibilities:

1) switch problems
Remove switch, clean with electrical contact cleaner (aka TV tuner cleaner). Spray, work switch, spray again, work it again, etc. Reinstall. To get access to switch, remove top and bottom covers at top of steering column, just behind the wheel. 2 screws for top, 2 for bottom.

2) bad ground at wiper motor
Yeah, I know, it's working on low speed. But bad grounds cause all kinds of odd trouble. Easiest thing is problably to add a new ground wire to one of the 3 or 4 small screws holding the cover plate onto the motor housing. Use the fattest wire you can manage. 14-16 gauge? The big mount screws go thru rubber bushings, so they don't create a ground by themselves. Other end of new ground can go to existing ground screw about a foot to right of wiper motor, it has a braided bare-wire ground going to it.
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Sven: '89 245 NA, 951 ECU, open-front airbox, E-fan, 205/65-15's, IPD sways, E-Codes, amber front corner reflectors, quad horns. Wifemobile '89 245 NA stock. 90 244 NA spare, runs.