Well I did alot of digging the past few days and found electrical schematics and documentation over at https://www.gmupfitter.com/pdflists/view/39
But the news is bad, basically the radio and VCIM (onstar module) are controlled by the BCM (body control module) via a serial interface, it is not just a 12v acc on wire that turns them on/off like the good ole days. So from what I could tell it would basically take tricking the BCM into thinking the key was in and in the accy position.Pretty sure that is not easily possible. so looks like calgator73's idea with plugging in and leaving the key in is about the only option which I wanted to avoid.
If anyone had other ideas I'm happy to tear into things to test stuff out