You need to take off the door panels and take a look ...my right rear window never worked so I finally got in there and found that the PO rigged up a PVC pipe to hold the window up because the plastic cable retainer broke (as so many of them do). I found a free fix but you can get replacement regulators for less than $100.00 if it comes to that. Check out my build thread for the free fix
if you find that you have the same problem I did.
It only takes a couple of minutes to see what is going on, then you can work your way backwards from the door.
Hope that helps?