if it's crud not letting motor to turn youo should hear circuit breaker clicking if you get in a quite location and put your ear down close to wiper switch. my '68 got bound up by a piece of rust that fell down to end of motor shaft - just a real small piece. cleaned it and reasembled it and been fine since. if you need any help on how it works PM me - its quite clever. when you put motor shaft back in tie the brushes up with very fine pieces of wire and remove after you get brushes in - that was trickiest part.
'68 6-cyl coupe
Canadian Mustang "Big Kit"