I wasn't referring to tapping into critical existing circuits - with most sensors, I wouldn't take the risk of interfering with normal computer function. I meant an add-on sensor/circuit to detect vacuum, rotation, spark voltage, oil pressure, water circulation, air intake, noise, vibration, temperature or any of the other myriad of things that change state when an engine is 'alive' ...
No idea what you intend on doing, under what conditions its needed, and how critical things like start/stop time and continuity are. Maybe what you need does not warrant the effort or expense - or maybe it does. But detecting the current run/stop status of an engine, and using that signal to conditionally provide a 12V supply, is certainly possible.
