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