How's the motion stuff working? I mean, if cameras are motion sensing, how quickly can a multiplexer be woken up to record, or would that be always on as well? Always supplying power to multiple cameras and a multiplexer wouldn't be low power draw anymore, and invites a 2nd battery install (not trying to do this).
I Like the Soundman98's idea of using the 4 Channel CCTV Security 3G Mobile & iPhone View HD DVR from ebay, but like you said it's another box in the car. It would be nice to have motion sensors,( also used for parking assist) that would bring the carpc out of hybernation and automatically start recording, great if someone decided to try to take you car for a joyride, or someone hits your car why'll it is parked, great evidence to take to the police.