From the ClubGP forums:
Hey, for all of you who have bought your Grand Prix or other Pontiac used from a dealer or individual, and have had to change a battery or alternator or both, you've probably run into the problem of having your Delco Theftlock Radio displaying a LOC code on the display. The only problem is, you might not be the original owner of the car, and the person who owned it before may have not given the dealer their unlock code, or might have forgotten it...so here is the skinny on unlocking it. If you already know how, then that'* great, but if you know of someone who'* having this problem, share the info with them. Stop letting the GM dealers charge you $50-$60 or more to do this procedure that a five year old could do in less than five minutes!!!!! Here you go:
1. Press and hold presets 2 and 3 for about 5 to 10 seconds until you get 3 digits on your display. This is the first 3 of 6 digits you will need, so write 'em down.
2. After doing step one and writing down the code, immediately press the AM/FM button to get the last 3 of the 6 numbers you'll need, and write 'em down. (if you wait too long between steps 1 and 2 and the LOC code displays again, just start over).
3. Call this number, it'* a prerecorded message, so don't worry about having to talk to anyone...1-800-***-****. The voice will ask if you're calling for Chevy, Pontiac, Cadillac, etc... When you here him tell you to press "2" for Pontiac, press ------ and hit the # key. He'll ask you for the 6 digit code that you've written down. Enter that and press the * key. He will then give you a code, and the option to get additional 4 digit codes, but one should be sufficient.
4. Follow the normal procedure in your owners manual for unlocking the radio with your new code...press the hour set button on your radio until you get the first two numbers of the code, then press the minute set button until you've entered the last two numbers. Immediately press the AM/FM button once the correct code is entered and the code SEC or "secured" will be displayed on the display screen. Now, just turn the radio on, and your ready to roll.
There is an additional step you can take to remove all codes from the radio and start from scratch by entering your own personalized 4 digit code so that it won't get you in a pickle again! Just repeat step 4 and you should get a display screen that reads ---. This means the code is out, and you can now program your own code. Of course, once you receive the 4 digit code from the GM recorded message, just keep it in a safe place, because you can use that same code again and again should you have any more problems with it in the future. Good luck, and yes, it does work.