well, a toggle switch, right?
Like when you turn your key from AC to ACC, thats basicly just a toggle switch, right? Then you turn the key to engine start, hold it till it starts, then let go and it falls back to ACC.
Thats just Like setting a toggle switch to ACC, pushing/holding a momentary switch to start the engine, the let go of the momentary switch, and only the ACC toggle is active.
Then to turn the car off, you set the toggle to AC, and when you get far enough away, the car locks and turns off on its own due to proximity.