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