The 2000-05 Cavaliers and Sunfire use "Class 2" wiring for the radio. A lot of stereo dealers will try to convince you that you need that "special" harness for $100-$150 to make your airbags work, oil change light, etc... but they don't know what they're talking about!! The only thing the stock radio does (besides play music), is produce the warning chimes... so if you can live without the chimes (just be sure to look at your car when you walk away, so you don't accidentally leave the lights one!)... all you need is the Metra 70-2003 harness, for a Chevy Tracker or equivalent. The 70-2003T is the expensive T harness that re-locates your stock radio to the trunk, so you still keep the chimes.
I found my harness at WalMart... I used the Scosche GM-035 and it worked fine.
The two things you need to do.... find a switched +12V signal (either from the windshield wiper fuse, or the HVAC panel brown wire, or the ignition column thick brown wire) to turn the deck on and off.... Class 2 stock decks use a data wire to "tell" the deck to turn on and off... there isn't a switched wire.
The other is a GM antenna adapter. The stock antenna has that tiny little plug... so you need the adapter to fit it into your new deck.
There's an extensive thread I made years ago on www.j-body.org
about Class 2 wiring... look in the Audio forum.