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