the harness is already availble. if i'm not mistaken, most of your vehicle is a colorado. the engine and electronics. they just tweaked it to do what a hummer can do. in fact, you'll find that a lot of your electronics is compatible with the h3.
soo... with that said, the harness for a colorado would work.
to keep your onstar you'll want a pac interface. click here:
http://pac-audio.com/products/onstar.htm.