Hello, not to disagree with the previous expert, but your 2004 radio system does not unlock like some of the earlier models. The only way it can be unlocked is with the use of a class 2 scan tool like the dealers use. A regular code reader can't do the job as they can't access the correct control modules. Below is a quote out of the GM service manual for your car. Thanks
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.