Hopefully someone with a lot more electrical experience will chime in, but it seems like you could go to any electronics store and pick up a rheostat of sufficient capacity to handle the load of the wiper motor in continuous operation, and wire it into the wiper circuit. Not sure where you would find out the amperage draw of the wiper motor, or how this converts into watts. Perhaps someone at the supplier site could help. I see Newark for example has a “Live Support” button available.
Something like this:
http://www.newark.com/jsp/search/pro...FC-GB100000001