disconnect stalk with motor connected and see if it stops it, indicating stalk at fault.
disconnect wiper relay box mounted on lh side of engine bay not far from motor.
if neither of these stop it, wipers have a brass ring with a cut-out inside. when you turn the switch off midway through a cycle power can continue to flow through the ring until the cut-out comes round and the motor will stop in the correct position at the bottom of the window. there could be a fault internally with this.
disconnect motor, if you find which of the wires is the constant power to the motor and run a temporary positive to this terminal. if the motor turns then the fault is internal. if it doesnt then its prob in the wiring.