Try removing the Deck. Get the serial number from the back. Call Vauxhall, they might give you the code.
That's how it works with other manufacturers.
I also heard you can put the ignition (not the CAR) ON for two hours until it clears. ????? Post back if it works.
This Forum is for BMW's but maybe they can help.
http://forums.bimmerforums.com/forum/showthread.php?t=406217
shtml54, August 2007