If you are wanting to run non-factory parts, then I suggest you try something like a Car Sound installation shop.
They might know a way you can do it.
I can't see how you will do it, because when you select reverse, a signal goes to the BCM, the BCM tells the radio to display rear video.
The communication is via a low speed CAN-BUS.
I would think it may cost you about $2000 to use factory parts, and I doubt if it has ever been done by Holden.
I'm sorry, but that's how it is.
How will the radio know if you are in reverse or not?
You want normal radio screen function until you select reverse, so the factory input to the radio to tell it to do this is the LSGMLAN (CAN-BUS).
I wish you well in attempting grafting another device in, and hope you don't blow something up.