Not sure if this will work on the B-Body line of cars but, give it a shot.
Taken from whitebird00 over at ls1tech referring to 98-02 monsoon stereos, don't know if it'll help you or not...
"TheftLock is designed to discourage theft of your radio. It works by using a secret code to disable all radio functions whenever battery power is removed (or when the radio is removed).
You usually don't have to worry about the TheftLock feature. TheftLock is not activated when you buy a new car and most owners don't bother activating it after taking delivery. Considering how little used Monsoon head units cost, there isn't much market for stolen ones. You can tell if TheftLock is activated by the flashing red LED on the face of the radio when the car isn't running.
When TheftLock is activated, the radio will display LOC to indicate a locked condition anytime battery power is removed. If your battery loses power for any reason, you must unlock the radio with the secret code before it will operate.
Unlocking the Theft-Deterrent Feature After a Power Loss
If you know the code that was used to lock the radio then use the following instructions from the owner's manual to unlock the radio.
Enter your secret code as follows; pause no more than 15 seconds between steps:
1. Turn the ignition on. LOC will appear on the display.
2. Press MIN and 000 will appear on the display.
3. Press MIN again until the last two digits agree with your code.
4. Press HR until the first one or two digits agree with your code.
5. Press AM-FM after you have confirmed that the code matches your secret code. The display will show SEC, indicating the radio is now operable and secure.
If you enter the wrong code eight times, INOP will appear on the display. You will have to wait an hour with the ignition on before you can try again. When you try again, you will only have three more chances (eight tries per chance) to enter the correct code before INOP appears.
If you don't know the code that was used to lock the radio then you will have to use the dealer's unlock procedure as follows. The dealer code changes over time - the one listed is current as of this posting. If it doesn't work try 08359 or search the forum for messages with a more recent code.
1. Press and hold presets 2 & 3 for about 10 seconds until the first three digits of the display code are displayed.
2. Write the numbers down.
3. Press the AM-FM button, write down the next three numbers.
4. Call 1-800-537-5140. When asked, enter dealer code 202108, press the * button, and enter the six digit code from the radio. You will get an unlock code, write it down.
5. Go back to the car, press the HR button and set the first two numbers, press the MIN button and set the next two numbers, then press the AM-FM button. The display will show SEC, indicating the radio is now operable and secure.
To remove the code, repeat step 5. you should see --- on the radio face indicating the code is now out of the radio."
So I have a 94 TA that has been in storage for a while.The radio locked up and I had been unable to unlock,UNTIL, I read this thread.
So here I am.I tried the advice above and I left the key on for an hour.It finally gave up the six numbers as suggested.I tried the 800 number and cant seem to get anything?It asks for a division digit.I tried 1 and 2 and it does nothing.If I enter the code given above it says invalid.What am I doing wrong? The digits on my radio that I get are 912925. Help??
"Da Factory Rat"
95 DCM Caprice LT1