I'm trying to do the same thing and it seems to me options are pretty limited if you don't have a CD Changer/Aux input.
The best option if you want to keep the stock head unit seems to be adding an aux input to it, like this:
FACTORY REMANUFACTURED PONTIAC FIREBIRD 7 EQ CD RADIO - !! MP3 INPUT !! - | eBay
I emailed a couple of people selling them asking them how they wired it up, so far no reply.