HAcking stock GM radios

Ok, It got a great idea from a friend of mine. He put a newer GM radio in his company vehicle to get the XM radio he bought working better. It got me thinking, How about searching for one of those GM radios that had the external Tape deck? can that be hacked to give me line-level inputs? Luckily Pontiac used such a stock CD deck in the late 90's and through 2000. I need to find a source for one of these that is unlocked so I can try and figure out how the tape input can be triggered and feed it linein for a mp3 player input.

Heck, having a stock radio means you dont have to lock your car as nobody will want to steal the radio, so the fiero can sit on a trip outside a resturant with the windows down and the sunroof out without fear.

More details as I get the parts!