Easiest way is add a Blaupunkt stacker in the glovebox - this is a dealer fit option and Holden can do this (although I would do this myself - labour is too pricey there).
Stacker plugs right on into the Blaupunkt factory unit.
I just looked at the Blaupunkt Oz site and matched my factory unit (it's the Blaupunkt CD Stereo 520) or your dealer can quote too (maybe AMCAP or a Holden wholesaler too).
If I was to do a new system (like I used to have in my Mazda) I would do a Sony MP3 CD 10 stacker under the seat, and an Integrated Sony head. Easy. You will need an interface to the remote control, but you can plug into the remote wires after unplugging the factory head.