"Aftermarket" is a term used to describe accessories that are not made by the factory. So for example, for your Plymouth Belvedere, if you buy accessories for your car that are not actually made by Plymouth, then those are termed "aftermarket". Many people use the term "Belvedere aftermarket accessories" specifically when referring to custom accessories for the Belvedere, like performance accessories or custom appearance accessories. But even if you just replace your Belvedere brake pads with OEM-grade factory-style brake pads that are not made by Plymouth, those too are aftermarket Belvedere accessories. Over the course of a vehicle's life, it almost certainly will have aftermarket accessories installed in it at some point, even if the vehicle is never actually "modded out".