I HAVE PUT FORWARD PROBLEMS WITH REAR WIPER MOTOR HERE BEFORE,,,,,,,,,,,,,WASHER FLUID ON WIRING (BOOT) ETC.EVENTUALLY MOTOR GAVE UP COMPLETELY.REPLACED IT YESTERDAY WITH OEM PART.WORKED ON SINGLE WIPE AND INTERMITTENT.ON STARTING IGNITION NEXT TIME , WIPER SWITCH OFF.......WIPER OPERATES ERRATICALLY!!!!!!!!!WHEN IT DOES STOP IT AGAIN OPERATES NORMALLY ON SWITCH >>>>FOR A PERIOD

CHECK UR PLUGS GOING TO IT WHERE IT GETS SO WET THE PLUG FURRS UP JUST TRY AND KEY IT BACK UP WITH A SMALL FILE OR SAND PAPER SO IT MAKES A DECENT CONNECTION

oooh, cap-fest! , but as Ninja says check the contacts are clean on the plugs as if you have had water ingress then corrosion can occur.