Rather a long time ago there were areas of the car market not occupied by the OEMs. How about a nice bit of plastic for your car, sir?
This advert is from the 1992 Daily Mail Motor Review. The back pages of car magazines usually featured this kind of thing. After you bought your car you could get rubber mats, car seat covers (the loud, tweedy ones were best), sun roofs and moon roofs, engine additives and car covers. Fog lamps could also be added, the more the better.