I see your point.
I've also worked out that a relay isn't going to work, because when the car's running even if the arduino is controlling it, the "on" wire from the key barrel is going to be hot.
I really can't see any way around this except to add a 10/5 minute delay on the arduino for the car to remain on for, in the chance that i don't get to my car in ten minutes, then bad luck i guess.
Cheers guys 