You will need to take your car to gm so that they can unlock the replacement deck (make sure you take a reciept or some other proof of purchace with you). However you can put an aftermarket deck in and keep your steering wheel controls. The same company that makes the integration harness that allows you to keep your factory warning chimes also makes an adapter that would allow to keep the controls. just make sure you get deck that allows for a wired remote and is compatible with the adapter.
Current Mods: Eibach Pro-kit springs and anti-roll kit, KYB Excell-G struts and shocks, Moog sway links, Prothane Front Control Arm Bushings (coming soon)