[JonJon68]

I forgot where I got this info but it helped me to unlock my delco radio in my 97 GPGT. Hope this helps:
1. First, press and hold presets two and three for a few seconds. Three digits will come up on the display. These are the first three digits of the factory unlock code.

2. Press the AM/FM button on the unit and another three-digit number will read out. These are the last three digits of your code.

3. Call 1-800-537-5140. You'll be asked for a dealer code, 139010, 206053, and 620529 may work. Enter this number and then press the pound (#) button.

4. Now enter the six-digit code that you got from your stereo, the first three digits first, then the last three. You'll be given a four-digit response code. Make note.

5. In your car, hit the HR button and put in the first two digits of the response code. Then press the MIN button and put the last two digits of your response code. Press AM/FM and then SEC should read out on your radio.

6. You should be unlocked!