rear wiper coming on by itself on starting car

Hi all,

This started a couple of days ago - I started the car and the rear wiper started furiously sweeping, the speed it goes when you spray the rear window. I assumed I'd knocked the stalk but it was in the right position. No amount of messing with the stalk made it stop, so I went and disconnected the clip from the motor. It seemed to just go continuously without any signs of stopping. 

I've seen a couple of threads about this maybe being related to poor batteries, but my battery is only a year and a half old and still very strong. There doesn't seem to be anything wrong with the stalk (although I thought maybe a microswitch might have failed inside?) and the wiring going into the motor looks ok, as does the motor itself - it's not making any noise or getting hot or anything.

Has anyone else experienced this and knows a fix? It'll probably be a garage job but if I can avoid that I'd rather, as I'm skint

cheers

Will