The sophistication of GM's theft deterrent system isn't generally discussed, and we may assume it's continually being updated/improved. For all I know, attempting to unlock a component that's been reported as stolen could silently trigger an On-Star report.
From what I've been able to glean, it's my understanding:
The GM theft deterrent system is made to inhibit the transfer of the entertainment system components not originally installed in the vehicle. Each component shares data with the whole system. And, like the internet, each unit has an identifier equivalent to an IP to facilitate this data sharing. When you install a component into the system, it's identifier must digitally introduced -- a "hand-shake" event. Unrecognized components must be identified and/or unlocked. With GM, the specific details of the unlocking process are not widely distributed.
If you browse the web, you'll find various sites offering unlock services from under $5 to $25 or up to what our dealer charged. They will instruct you to perform the necessary procedures to determine and enter the data specific to your installation. Typically, they'll tell you how to get some numbers that you'll send to them. And, by return eMail, you get the unlock coding procedure -- presumably pushing various combinations of buttons like setting the clock. This is necessary if you don't have access to your car's data link.
If you have an OBD-II scanner, then you can access the computer and you might be able to unlock directly. The typical procedure involves going to the unit's set-up menu and launching the Learn VIN routine. This is necessary when re-installing the original standard radio, and may also work for other component. I was unable to find info specific to your model.
Sorry your dealer wasn't able to help. In many areas, dealerships sub out what used to be called "stereo" work. To unlock a component, go to a speciality shop that installs and services the entertainment electronics for your local dealers.
