So, the correct answer is, "Whatever they want."
Sorry, you asked.
In my C-2, which is a DRIVER that I decided to buy after years of trailer-queens and a room full of dusty trophies, I put in a system against all the "rules." The C-2 world is composed mostly of people who "listen" to their cars, and I get it. I do too, sometimes.
However, on a longer drive, I like some tunes, so I bought, before the major price increase, a look-alike radio from Antique Automobile Radio in Tampa (really Palm Harbor, but close enough for people who do not know the state.) Current price is $700; I got it for less a few years ago, and it is the same unit.
Then, I was able to modify the Custom Auto Sound footwell speaker enclosures to fit my car, which has factory A/C (pretty rare--only 5% had it). So, when my daughter is with me, she can blue tooth her music through the system, and I have a plan to do something along those lines as well.
The daily driver (2007) needs a new radio--all of a sudden it is static-y when the engine is on. I don't know what I will do there yet.
Cheers!