First verify power is removed when the switch is off (open).
Most wiper motors have a park feature the run the motor after power is removed until the wiper arm is in the parked position. This requires an unstitched power wire, if the park contacts stay closed the motor will not stop once it gets to the park position. Resolution is replace the motor.
1997 2400 SCR
5.7 Vortec / Bravo 2