If your cars a 2003 your going to need the metra 70-2003 harness which requires use to run a new power wire to a switched accesory lead, to make it easy just use an add a circut on an unused line like the sunroof circuit.
or you could save time, but spend more on this
http://www.sonicelectronix.com/item_...R-GM09SRB.html, which keeps your vehicles chime system in tact
I have the scosshe, but personally if i had to do it again id buy the cheaper harness
In 2002 Gm changed the class 2 data system which changed the radio harness