On the wiper gearbox there is a single wire connector (light green according to RAVE) which supllies a continuous 12v to the parking switch built into the gear box. If this has 12v on it the it is the internal switch. On other wiper motors (haven't had to go there on the F/L) the cover can be removed by three screws to give acces to the switch and gears. The switch is open to gear box grease and may have become dirty.