It can be either or both. As Vrollvo said, the spot welds at the top corner of the backrest frame can fail, allowing it to twist out of shape. This will be obvious, as the seat will be grossly distorted, (and extremely uncomfortable to sit on) and it will flop around quite a bit. More common is deteriorated crumbling foam, which will cause a misshapen but still solid seat. It's not that difficult to take the seat apart and swap your skin over to an intact frame and foam, and, as previously mentioned, the frames and foams are interchangeable side to side. The trickiest part of the job is probably removing the big knob that adjusts the back rest angle. It is not obvious, but it has a round center cap that must be pried out, and then a sheet metal spring clip that has to be removed very carefully so as not to render it unusable. To remove the headrest there are a couple of buttons to push at the bottom of the tubes in the backrest frame that its legs fit into. You can't see them---you have to work by feel. You may have to cut the upholstery on the donor seat to get a good idea of where they are.