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