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