How do you fix a VW Jetta seat belt that is stuck. It is the rear passenger belt. I just bought another one from junk yard and it slowly retracts the belt and won't release?

The used seatbelt you bought may be broken. However, if it was working before you took it out it may be the same thing that happened to me. When the belt is not installed in a car it locks up. But once you latch it into the car it will push down a lever that allows it to work.