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