It's possible to splice into the stock changer wires so you can put the audio signal directly into the radio while still keeping the changer.
Click here to see that schematic.
For the install, if you get a receiver with a control unit that's small enough you can put in where the ash tray is and still use the door flap to cover it up. Check out
this thread to see what it looks like with a Sirius.
Whatever you end up buying, check out post #9 of the C5 FAQ sticky to see how you can power it all with RAP so it all stays on with the radio.