The first thing you need to do is to clean the outside of the wiper motor the best you can. This motor works based on the ability to form good vacuum seals, and that just can't happen if you have dirty seals.

Next, take a look at the air intake port on the bottom of the motor. If its plugged with dirt, then you'd found the problem. (Don't confuse the air filter just inside the intake for blockage--I'm taking about debris in the intake hole itself.)

If the intake port is clear, then you need to take the motor completely apart for cleaning, polishing and relubricating. Begin by removing all of the funny little screws from the outside of the motor.