hmmm, I think I found the key for my ignition/doors worked that switch? if not, I had to replace mine anyway.. bit of a pain but not the worst job in the world..
Not even $20 4 pin switch from jaycar.
but this still may not be your problem, i've had a switch fail on me before but yeah..
I'd also maybe check things like the crank angle sensor, or make sure its in Park or Neutral if auto and check the cable that goes to the shifter to determine if its in p or n.
I think it would definitely be worth doing a diagnostic test, which will show up error codes on the dash if there are any by flashing the engine light (as far as i remember) in sequences.
ALDL Information for the VN & VP Holden Commodore
Diag can be done by bridgin 'A' & 'B' in their imagine with a paper clip or something of the sort.
The codes come out flashing the engine light a number of times with pauses.. i've seen people explain it much better, but without going into a big rant.. e.g code 12: flash, 1.2 sec pause, 2 flashes. Thirteen would be the same just with 3 flashes instead of two after the pause.. 21 or anything above would be two flashes BEFORE the pause instead of one.
that link should give you a rough idea what to do, if you go down near the bottom it tells you what code is what. Good luck mate! let us know how you go!