No worries!
Would you be running the 6x9s from the head unit amps, or an external amp?
The speakers will just drop straight into the parcel shelf once you pop the punch-outs from the panel. (you'll also have to remove the interior trim on the parcel shelf too) Grilles which go into the parcel shelf trim are available from Holden, but look elsewhere too - I got mine from eBay for less than half of what Holden were charging. I had to cut the holes in the trim to fit the grilles, but that wasn't too difficult.
As far as wiring, I believe there are wires run to there but I've not identified them. (my shelf speakers run amp'ed) If you are running the 6x9s off the head unit though, you'll need to disconnect the back door speakers, otherwise there will be too much load on the internal amps and they might fail.
Cheerz!