this is a little late, idk if its too late to cancel your order but if you dont have onstar or anything else you can use this wire harness thats on amazon for a penny plus shipping Amazon.com: Metra 70-2003 Radio Wiring Harness For GM 98-Up Harness: Car Electronics this is the harness that i recently bought, have not hooked it up yet but from what the guys on here have told me is that it will work just fine, only thing you have to do is run a wire from the back of the headunit's power source to the passanger fuse box for a power source because the stock harness on 04+ grand prix's dont have a power source, for more info look at the thread i posted here. it has a lot of good info http://www.grandprixforums.net/wirin...ion-58530.html
