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