I have this too, think it's the doors rear central mechanism dragging on the externally applied spray grease used to get out of noisy trouble then not sorted properly, I believe VW recommend lube inside the roller area rather than on the runner.
Well it looks like the cover was moving shutting the door,i have took the cover off and the trim holding the cover on was slopping around and the wheel wasn’t running true along the track it is badly corroded so i am going to replace with a new one after removing rust and repaint etc.
Just replaced the nylon block and roller on mine. Door sliding along nicely now. Agree with @olnow - those 3 links should help you out. http://www.itinerant-air-cooled.com/viewtopic.php?f=49&t=9182 http://www.itinerant-air-cooled.com/viewtopic.php?f=49&t=9144 http://www.itinerant-air-cooled.com/viewtopic.php?f=49&t=9155
