Few adults feel the need to put in a custom stereo.
Do a google search on car stereo shops in your city and tell me that again. Out here in reality lots of people replace their stereo, certainly a high enough percentage to make turning the stereo into the check engine light a bad idea.
I've done it twice, both times was to put in a CD player versus a tape deck.
Once was on a 1989 Mustang GT (I had a ton of CDs, but few tapes), and the second was three years ago on a 1998 Jeep Cherokee XJ (CDs had been out for over 15 years, when this thing was made, but they still had tape decks on the base radio). Three years ago, my tape inventory was exactly 7 tapes, but I had a ton of CDs and an MP3 player, so I got the cheapest thing I could find that would accommodate both.
No kidding. My 4Runner has a fancy JBL stereo that I never listen to. My damaged hearing has a lot to do with that of course.