Trouble with wiper motor

Checking out the Duster prior to it's yearly safety inspection and I discovered the wiper motor does not work. It has worked in the past and since it is not driven in the rain and used very little, the motor is not exercised very much. I can turn on the switch and at the same time pull on the wiper arm, it will move a little bit on ots own.

What do I need to do to get it working properly again?

Thanks Gary