Here is a brief description of whats going on.
The radio has the THEFTLOCK feature if the display reads THEFTLOCK. This feature is designed to discourage theft of the radio. The feature operates by using a secret code in order to disable all the radio functions whenever the battery power is removed. If the THEFTLOCK is active, the THEFTLOCK indicator will flash when the ignition is OFF. The THEFTLOCK features use is optional. If not used, the system plays normally and the radio is not protected by the feature. If the THEFTLOCK is activated, the radio will not operate if stolen.
When THEFTLOCK is activated, the radio will display LOC in order to indicate a locked condition anytime the battery power is removed. If the battery loses power for any reason the radio must be unlocked with the secret code before it will operate.
If the security is activated and the radio is disconnected from battery power, the radio will display "LOC". The radio will not turn on until the correct unlocking code is entered. If the customer code is lost or unavailable, contact your local GM dealership or an authorized Delco radio service center for additional assistance.
So, by that being the case, lets try to pull the back up code. Follow this procedure:
If pulling this backup code is unsuccessful, then calling the dealership and giving them the serial number off the unit is the only other option. If you are successful in optaining the back up codes, follow this procedure to enter the code.
If you can not get it unlocked or you do not want to call delco provide me with the serial number and i will get the information for you.If you have any other questions please ask if not please accept pay and feedback thank you bonus always appreciated