"Aftermarket" is a term used to describe parts that are not made by the factory. So for example, for your Mercedes Slk-class, if you buy parts for your car that are not actually made by Mercedes, then those are termed "aftermarket". Many people use the term "Slk-class aftermarket parts" specifically when referring to custom parts for the Slk-class, like performance parts or custom appearance parts. 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 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".