Same exact problem happened to my '85. Like Ken said, the ribbon is stripped. Cheap fix but definately takes some patience. I have no idea what a dealer would charge to fix this. I did mine myself with a little patience. You have to remove the screws holding the door panel together. You have to remove this to even see what you're dealing with. The glass can stay in the car, but will have to be all the way up via some kind of wedge. Then remove the screws that holds the track for the ribbon (you will now see that it is stripped). There will be one pop-rivet at the very end of the track that will have to be drilled out......i replaced the rivet with a little nut & bolt. Then replace the ribbon & put everything back together. Kind of a pain in the butt, but it can be done at home. Hope this helps.
Jay