If its whirring it could be the wheel boxes or the rack worn or broken.Take off the plastiuc cover over the wiper motor and the small bolts holding the ally cover.Underneath the grease you'll see the connection to the rack (a sort of wire toothed cable that goes to the wiper arms). It might have broken or coe off the pin,if not if you disconnect it and turn it throught 180 degrees and reassemble it might work as you then use an unworn bit of rack (unless of course it's been turned around before).