Many of the Holden Dealers are now giving that info out free of charge in a more friendly approach of "Public Relations". Only if it is a newer vehicle are they more anal about ownership identification.
Here in Adelaide two dealers give out this info much more freely and don't take the stance of screwing their owners over with such a petty thing that takes all of 15 seconds of their time to retrieve.
One thing to take into consideration though is whether the radio is the original unit fitted, if not then you've got no hope.
The only other way is to contact Eurovox who made most of the original units and give them the radios serial number and for a massive fee they'll give you the code. Don't know anyone that's done this though as it's just way too expensive.
I love Toyota's method of using the last 4~6 digits of the cars VIN number as the security code that's usually entered by the original selling dealer as their radios have selectable security code procedure in them from day one.
