As much as I hate the thought or having to do it, I think you are right, I need to create a new line from the battery with inline fuses - I'm just not looking forward to running through the firewall
Current draw will be small, its only an Atom processor in the PC - current draw should be low enough to be in-sleep mode all the time its not in use, without having to shut down the PC - hence I need an 'always live' feed. The screen with be on a different circuit, and will only come on with ignition etc.