A few more details and a picture would help us to offer specific advice.
The inside rear quarter panels below the rear window typically curl up from heat coming through the window and because they are flat. This is much worse if you have leather, as the leather shrinks substantially, but the vinyl covered ones also curl, just not as much - at least in my experience.
The first job is to remove the panels, which will require you to remove the rear seats. Once the panels are removed, you can peel back the vinyl or leather covering, and the foam below to access the panels themselves. They are comprised of a sort of moulded wood/cardboard/resin composite. They can be flattened through the application of heat and pressure, and or by adding some metal plate or other strengthener below the curled area. Interestingly Porsche added fibreglass reinforcement in some other areas, but not in this problem area. There are a variety of other methods, but this is what I did on my '88 - there is a description and pictures in the profile for my '88 starting here:
928oc.org/forum/928-oc-member-car-profil...in-s-88-s4-auto#3087 
