This happened to my brother's 89. We chased the problem for over a week, and about ended up in the nut house. A new wiper motor did the same, a replacement switch did the same. Finally, Look at the wiring harness where it plugs to the wiper motor. It can corrode, or even loosen up. I figured out how to get each wire out of the connector one by one, and cleaned and squeezed the connector back together so it would fit tight on the pins on the motor.