2000 GC.

The rear wiper on my van is selective on when it wants to work. In the morning I'll use to wipe the condensation off the window and get down the road and I hit it again to wipe the snow off the window and nothing. I've noticed toot that sometimes if I grab the wiper arm and wiggle it in and out and back and forth it will start working again.

 

Check the electrical connection to the motor and the ground. It may be loose or corroded. See if you're getting power to the motor while you're at it. If all is good, replace the motor.