Holden radios unlocked online — from the Bosch serial on the unit, or from the VIN on GM-built units.
Holden shared its head units with the wider GM range, so two doors lead to the same answer: the Bosch serial on the casing, or the car’s VIN.
The VIN is on the plate at the base of the windscreen, on the door pillar and on your registration papers.
The serial is the surer of the two, because it names your exact unit; a VIN names the car, and a radio swapped at some point would give the wrong code.
Neither rule has been confirmed by a Holden owner yet, so the code is free: a share unlocks it and we ask you whether it worked.
Serial number example:
0400512345600
17 characters, no I, O or Q.
W0L00000131149563