i can see your logic in using just the motor part of a windscreen wiper motor , but really , your not going to get much better than the gearing it comes with , as said before, its going to very hard to mount , and other better , and possibly cheaper motors are available.

Ive never personally used them , but they look like they should be perfect for what you need

http://www.technobots.co.uk/acatalog/On ... rs_76.html

1st and 2nd one