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