Is this of any use?
If the screws just turn, its likely that the treaded inserts have become loose from the body panel itself. They are a sort of 'crimped' fit. You need to get access from behind by removing some of the interior trim (if you have a boot mounted CD changer, the side panel is already designed to come off.) You can then hold the threaded insert from behind with pliers whilst undoing the crew from the outside.
The insert can be refixed with a blob of superglue and make sure the screw is lubricated before retightening (not too tight though!).