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