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