Sorry I'm not around as much as I used to be or I could have helped much sooner. We see a lot of water intrusion into wiper motors that make them behave that way. I have a Vette in my stall right now where the cowl drain tubes got clogged with leaves and filled the cowl with water and submerged the wiper motor. Usually a tell tale is when you disconnect the wiper motor connector from the motor, it will show corrosion.