There are two circuits in a wiper motor/gearbox -- the main power circuit and the "park" circuit, which runs through a relay in the unit that is energized when the main motor is in operation. A sticky park relay can send power to the wiper motor when it's stuck closed, bypassing the stalk control, or it can stick in the open position, in which case the wipers stop in mid-sweep. I recently replaced a motor on a BMW that had a bad park relay that was stuck in the open position. The wipers would not complete a full sweep in intermittent mode.

You can test the motor circuitry with a multimeter if you can get to the connector. That's a good way to test the switch on the steering column as well.