WHOAH.
Aftermarket radio is NOT NOT NOT the cheap or easy way.
FM modulator is the absolute best way.... get this. Dont listen to that "FM modulators sound like crap" nonsense... they sound fine. I have one in my Aurora and it sounds just as good as a CD.
http://isimplesolutions.com/ipod-adapter/TranzIt.asp
Costs like 70$ or so on ebay. Then get the 2 antenna adaptors you'll need on ebay (small to big and big to small), too. Finally get a pair of RCA cable Y splitters so you can but both the Aux jack and the Ipod cable in.
Then you dont have to fsck around with a new accessory wire and losing chimes, or paying for the GMOS harness, or anything.