It is all designed to prevent battery drain, even the radio goes off after a couple of minutes when ignition is off. It is also monitored by the canbus/battery management so you may get a fault code if it detects unusual battery drain when off. having said that, there is an always on 12v supply to the rear view mirror (for remote control) although putting a socket up there might look odd. I don't know of any other 'always on' supply in the cabin so yes, would need to route to the battery which is no big deal.