how bad is it? I'd have thought you'd just be able to get a bodyshop to repair the area rather than replace the entire rear quarter. I don't think the panel is available but you could just cut a clean one off in a scrapyard. However, I think it's highly unlikely a bodyshop would want to go to that extreme unless the damage is that bad.
A mate of mine bought a mk2 coupe s once which had a LOT of rust around that area, the bodyshop just cut the rust out and made plates to replace the metal and skimmed it with filler, looked as good as new afterwards. Was only about £300 if I remember correctly.
