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