get yourself the cd radio for these cars, they're reasonable on ebay.
For mp3's. look at spending 100-200 dollars depending whether you have an ipod or not.
go to Cardomain.com and order the GMH32T harness that adds a cd changer line to the factory stereo. then get the harness for the pac aux-box, that converts the mp3 audio signal into something the radio can understand. This setup is about $100
If you have an ipod, you can get the pac aux-ipod box, that used the ipod's special connector, and lets you control the ipod with theradio's controls this setup is about $200