It could be bad brushes...what year is the car?
It could also be just plain old lack of lubrication. I'm not experienced with the mini wiper motor, but if it's like many others, you can remove the top plate of the worm gear and freshen-up the grease.
The cable may also need some lube. Molycoat is a graphite based grease that I'd be inclined to use in this application.
On their death bed, nobody ever said, "Gee I wish I'd spent more time at work!"
'68 Sprite 1275
'76 Midget 1500