"Aftermarket" is a term used to describe accessories that are not made by the factory. So for example, for your Chrysler Imperial, if you buy accessories for your car that are not actually made by Chrysler, then those are termed "aftermarket". Many people use the term "Imperial aftermarket accessories" specifically when referring to custom accessories for the Imperial, like performance accessories or custom appearance accessories. 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 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".