WelcomeCustomer The dealer was not entirely accurate. The radios in the newer vehicles no longer use a code to unlock the radio, rather, the system has to be programmed to the vehicle using GM's scan tool. This process learns the vehicle information into the radio. This only takes 20 mins and should only cost $50 or less. Below is the section from the manual that describes the Theft deterrent of these vehicles. Please ask if you need more information.
The Theftlock theft deterrent system is a feature on all base and uplevel radios. The theft deterrent system utilizes GMLAN serial data to determine if the radio is in the appropriate vehicle. Each time the radio receives the vehicle ID (VIN) packet message and is in the RUN power mode, it compares the VIN information it has stored to the received VIN packet message containing VIN information. The radio receives this message from a module on the GMLAN serial data circuit responsible for transmitting that information. If a mis-match occurs, the radio display will indicate to the user the radio is locked. Once this takes place, the radio will not respond to any button presses and become inoperative. The 2 scenarios able to cause this condition are:
The only way to unlock the radio is by using a scan tool. The scan tool sends the appropriate diagnostic message (relearn VIN) to reset the radio. Using a scan tool, select the radio VIN relearn procedure from the module replacement screen in Vehicle Control Systems.