99 Camry LE radio

My radio is a single cd, AM/FM, w/ cassette. The first 2 preset buttons for the radio are labelled Disc 1/Program and Disc 2/Set. As far as I can tell they've never done anything when I have a cd in. Does anyone know what these are for if it's only a single disc deck? The ideal thing would be that they can be used to switch between an AUX plug and the cd player, but I highly doubt that's the case. Just seems odd that there are use buttons on my radio.

I want to get an AUX input upgrade for it so I can plug in my MP3 player (which is not an IPOD), but everything I've seen about them says that you have to unplug the cd player in order to use it and I'd like to keep it up and running if at all possible. Getting a new radio is out of the question right now since the only ones I've been able to find with an AUX jack in the faceplate are over $150 after installation.