Hard to find datasheets?
 

More importantly, OEM equipment datasheets.

I'm modifying my stock CD player(well, tape first to prove concept and then am going to modify the cd player). The reason for this is to add extra inputs while staying with the stock look, mostly. Once you start the car you'll notice that there isn't typical Vacuum Florescent Display behind it but a dot matrix one. How many people would notice that I don't know.

The stock head unit has zero aux inputs and I plan on adding about 3 of them. One for the mp3 input, one for the XM, and another for whatever I decide to add later. I've thought of making it HD radio but the cost and learning curve of it all will be insane if I add too much.

Just look up chips until you ID the amp, look up the reference circuit design and I bet it comes close.

You might check to see if a Sam's Photofact set is available for your stereo.