got this from CGP
So here's the free procedure on how to do this...
1. Press and hold the preset button 2 & 3 or 1 & 4 for older radios, about 6 to 9 seconds Until the first 2 or 3 digits of the display code are displayed.
2. Write the numbers down.
3. Press the AM/FM button; write down the next 2 or 3 numbers.
4. Call 1-800-537-5140
The recording will ask you to enter a number corresponding to a subsidary of GM ie, press 1 for Chevrolet, 2 for Pontiac...Ignore this and enter the following number 106010 and hit the star button *. Now enter the numbers you obtained from the radio itself followed by the star * key, and in mere seconds, you'll have a valid unlock code.
5. Use the HR button on the radio to enter the first 2 digits of the unlock code, and the MN button to enter the second 2 digits of the unlock code. Now press AM/FM and you're done. Unlocked radio!
ClubGP Message Forum troubleshooting