MP3 players came out in 1998, the design of the head unit in our cars goes back before that - remember, the sound system of our cars was outdated when the cars were sitting on the lot.
There is a mono audio input for the old-style in-car cell phones, but that's it. As to the multiple CD changer interface - that is a different format.

Look at one of the Kenwood, JVE or Clarion heads, you will get what you want, plus bluetooth and other cool stuff. And by running four more wires to the head, you can have four channel sound in the car, too!