There are some posts regarding what can be done with the factory stereo. I believe if you remove it completley you will lose the steering wheel functions to control the stereo. What has been recommended is to leave the factory stereo hooked up and left behind the dash (apparently there is room) and then install the aftermarket deck.
I hope this helps a little.
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03 Hummer H2 Adv./Lux. Pkgs. Sunset Orange W/Sunroof
Air Spring Susp. Pkg. + 3rd. row seat
http://www.haugens.com