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