I found this information for you, after a little surfing. I hope that it helps. I had an aftermarket stereo in my Tracker at time of purchase and I don't remember anything about the stock radio that I had in my old Kick, because it didn't stay in there fo very long.
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There are currently two different versions of the TheftLock® system that have been used in General Motors vehicles.
VERSION ONE
The first and earlier version works by using a security code made up of four numerical digits to disable all radio functions whenever battery power is removed, such as when the radio is unplugged. On this system, the TheftLock® feature for the radio may be used or ignored. If ignored, the system plays normally and the radio is not protected by the feature. You can activate TheftLock® by inputting your security code. The radio will not operate if stolen. With the ignition off, the TheftLock® LED indicator will flash, indicating a secured radio. The red LED is located on the face of the radio. When TheftLock® is activated, the radio will display LOC to indicate a locked condition any time battery power is removed. If the battery loses power for any reason, you must unlock the radio with the security code before it will operate.
If you are the second owner of the vehicle, you may be unaware of the TheftLock® feature, never the less that there is a code. Many owners and/or dealers write the code on the first or last page of the owner's manual. If you have the code, you can follow the direction in the radio area of the owner's manual to input the code and unlock the TheftLock®. If not, you will need to visit the dealer to obtain the factory code from Delphi Delco Electronics. At this time, the customer may elect to disable the TheftLock® feature so that they do not have the condition reoccur. Most dealers will charge for this service.
VERSION TWO
The second and newer version of TheftLock® is also designed to discourage theft of the radio. The feature works automatically by learning a portion of the Vehicle Identification Number (VIN). Therefore, you don't need a security code to activate the system. If the radio is moved to a different vehicle, it will not operate, and LOC or LOCKED will be displayed. If LOC or LOCKED is displayed, the TheftLock® system has locked up. You must return to the dealer for service. The only way to unlock the radio is by using a scan tool. Most dealers will charge for this service.
