Well, you've got 2 options:
1. Hook up the Opus to the ignition, and turn the ignition on to use the computer.
2. Hook up the Opus to a switch somewhere, so YOU can decide when you want it on :) This is what I do, and it works real well. If i'm just moving the car out of the drive for my mum to get her car out, the computer won't boot, or if i'm popping up the road to the shops, it won't turn on, or i can sit there while washing the car, with the computer on, without the keys in the ignition :)
Also, with the Opus, you have the advantage that once the PC is on without the car running, if you DO need to start the car...the computer won't reset itself! ;)