Even after car radios were a bit more decent, usually AM/FM and featuring cassettes or CD, there was a standard perception that you installed aftermarket stereo because all manufacturer’s stereos were crap. IIRC, that persisted about into the 80s, and there was a large market for theft-deterrent stereos with lockout codes, removable faceplates and so on. About then, car makers started paying attention to the quality and features of their stereos, and making them a selling point. As well as beginning the process of customizing them to the car, so that now you can operate the stereo from buttons in the middle of the steering wheel in some models, or see the display from the radio on your dashboard.
I’d say well into the 90s. I started driving in the early 90s and pretty much every single person I knew, new cars or used, had an aftermarket stereo installed. In fact, a good portion of my friends still have aftermarket stereos, and I can see why. My 2005 Mazda 3’s stereo is a piece of shit if you really care about sound quality. I’d get an aftermarket unit, as well, but I don’t feel like giving thiefs another reason to break into my car.
It took me 5 minutes to figure out what this meant.