I think what you are describing is the RAQ radio. The simplest would be to buy an adapter. You'll need one of these depending on whether or not you have the satellite radio option: http://www.logjamelectronics.com/piechryaux1.html
This particular adapter is 80 bucks; it was the first to come up on google. There are probably others, maybe cheaper. Just search for RAQ input adapter, etc.
You'll also need a 2 dollar cord to go from rca to 1/8" stereo - http://www.amazon.com/3-5mm-Mini-Plu.../dp/B000FEHO0U
As far as downgrading the radio, you probably could do that without a problem. However, the adapter would likely be cheaper; and you'd keep the mp3 cd changer capabilities.
Finally, the cheapest and most difficult option: I have heard of people tapping into the wires on RAQ radios that are compatible with the handsfree or satellite radio systems. These evidently have audio inputs in the harness already. You'd have to get the wiring diagram to ascertain the exact locations.