Just to add to mrweather's great pictures:
Have a look at this iPod cable installation.
The plastic panel (in this case the right hand drivers side) unclips allowing access to the rear of the radio unit.
It is possible to carefully fish cables through the holes shown.
The cubby hole has a removable plastic liner which you pull out to expose the holes.
The Pontiac G8 and Holden Commodore interiors should be identical except the drivers position of course.
As I have the Sportwagon version, running the antenna cabling has proved to be a bit of an issue.
What I am going to do is identify a gap in the side of the tail light mounting, make a hole in the side and feed the coax through into the vehicle body along with the internal wiring harness.
Tail light assemblies can be bought cheaply, so the intention is to get another one and make the modifications, seal it up neatly with silicon and the result will be invisible and water tight.
As mrweather has shown, the sedan trunk lid is the easiest mount to- as long as you drill those holes!
Commscanaus.