I am inclined to think it would be a "CD Changer" - multiple CD's (usually 6, 8, or 10) loaded into a magazine. I think I've heard it called that before...
You're probably going to end up having to change out your headunit - since CD Changer controls are typically inbuilt into the thing, and the interface cable that provides control also provides a signal path, so that the CD Changer can use the in-dash headunit as a preamplifier and an amplifier.
Considering that the attachment point for that interface cable is typically on the back of the headunit (I've installed a few...) you'd need to dismount the thing to check - and then find a compatible CD Changer (usually the same brand.)
You may have an easier time just changing out the headunit for one that is compatible with a CD changer already - and, if you're into .mp3 collections, you might also want to get one with a "front panel input jack" (typically 1/8" stereo headphone plug/jack) or get one with a USB port that will accept a "thumb drive" with .mp3 files on it.
(I'm handing you excuses to upgrade - get it?)
For a jumping-off point, go to
www.crutchfield.com. I've actually used them quite a few times in the past, and I've gotten good service from them - but they've got an index of what will fit your rig, and you can take the information you get from them and cast about for price, if you care to.
Considering you don't really have a place to "secure" a CD Chager out of sight anywhere, I'd go with the front panel input approach, and use .mp3 files instead. Or, get one that can read .mp3 files off of CD (that's also out there,) and you can burn CD's that last a good six times as long as a Redbook audio CD anyhow... mp3 file compression is miles more efficient than Redbook anyhow.
