Sweet, that sounds like the way to go. Yes it's just a normal 2-din headunit for sound/cam/nav. It didn't control the HVAC, just a displayed it. A bit pricey so I didn't want to risk hurting it.
I've now tried that with the camera, but no it doesn't auto trigger at all

Nice thought. Handy having ciggie lighter power cords for everything, so one fitted perfectly for testing. Cut the plug on another for testing things too, so it's easy to just power up anything 12v and it's fused. I kind of like having it powered to the accessory anyway so I can switch it on whenever. Really handy when towing to keep an eye on the load (not that I put a towbar on this car), or seeing how close those tailgaters really are. But would have been a handy solution.
So yet another trip to that dirty old man at jaycar is on the cards then. Lucky I'm a dirty old man too, so it's not bad. I think he likes seeing me so he can tell his jokes. Just looked through the catalogue and it looks like they have exactly the right regulator, 9v even. Appears this will be the perfect solution.
So thanks heaps for doing that thinking for me (with this insomnia I needed it, as you can tell). Really appreciate the help.