"Aftermarket" is a term used to describe parts that are not made by the factory. So for example, for your Plymouth GTX, if you buy parts for your car that are not actually made by Plymouth, then those are termed "aftermarket". Many people use the term "GTX aftermarket parts" specifically when referring to custom parts for the GTX, like performance parts or custom appearance parts. But even if you just replace your GTX brake pads with OEM-grade factory-style brake pads that are not made by Plymouth, those too are aftermarket GTX 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".