Article from Ray’s Chevy Restoration Site - http://rmcavoy.freeshell.org/electrical.html

Q - How is the windshield wiper motor wired ? How can the circuit be tested ?

A - Single speed wipers were base equipment on most early 60's GM vehicles. They have one (fused) power wire that is connected directly to the wiper motor. This wire is live whenever the ignition switch is on. A second wire connects the motor to the switch on the dash. When the wiper switch is turned on, it completes the circuit to ground (the dash). The single speed motor can be tested using the following procedure:

Two speed wipers with washers were available as optional equipment on early 60's GM vehicles. They became standard equipment in 1965 or 1966. This system still uses the same basic wiring principles as the single speed wipers. There is still a power wire that feeds the wiper motor whenever the ignition switch is on. Except now there are three wires going to the switch. The switch grounds one wire for high speed operation. Two wires are grounded for low speed operation. The third wire grounds the washer solenoid to activate the washers. Note that many '75 and later vehicles used a small electric washer fluid pump mounted to the reservoir instead of the older wiper motor driven pump.

There are also 3 types of 2-speed wiper motors. One is a non-depressed park motor that is easily identified by its rectangular motor case. Another is a depressed park motor that has a round motor case attached to the gearbox at an angle. The third is a permanent magnet motor that has sort of a rounded 6-sided motor case. The diagram below illustrates the non-depressed park (rectangular case) motor. The washer has been omitted for clarity. The depressed park (round case) motor uses the same wiring concept except the terminals on the motor are arranged in a different order (3 Low, 2 Power, and 1 High). The permanent magnet motor (used primarily on 1978 & newer GM pick-up trucks) has separate terminals for the park switch and therefore uses a different wiring setup. I'll add a diagram for that later.

Testing the wiper motor independent of the switch:

The depressed park and non-depressed park 2-speed wiper motors can be bench tested independent of the dash switch by making the following connections:

If the motor operates normally in the above tests but not when installed on the vehicle then the probelm is most likely in the switch or maybe the wiring. Don't forget to check the fuse. The switch can be tested with an ohmmeter.

If the motor stops immediately when switched off (doesn't return to park position), first check the motor's ground strap. Since the motor is mounted on rubber cushions, it's grounded via a copper strap attached under one of the mounting screws. This is the ground for the park switch so the parking feature won't operate if the ground strap is missing, dirty, or corroded. If the ground strap checks out okay but the motor still doesn't go into park then the problem is most likely a worn out or dirty park switch. The park switch is located inside the motor's gearbox so some disassembly is required to check/clean the switch. I recommend consulting a good repair manual for the motor disassembly procedure.