From what I remember, old wiper motors take a good 10 amps steady and possibly 10 time that much when starting up. I suspect that you will need to run it off a 20-30 amp battery charger. The cheapest way is to rewind a transformer from a dead microwave.
Are you sure your pwm controller can handle such large currents?
Why don't you use a variable speed electric drill instead of a wiper motor?