When my Neon kept getting stolen, I looked into using a crank position sensor from a Neon which is a latching sensor. Mount it in the dash and pass a magnet over it to turn on, pass again to torn off.
Thought about a magnetic switch with a circuit that was open if the magnet wasn't in place. Something hidden next to the seat, and a fob that could be placed in the right spot when you got into the car kind of subtly. Then I decided that I didn't want to have to order new ID and credit cards every couple of weeks because the magnet kept erasing them.
I know there are RFID kits out now, too. Seems like it would be a similar arrangement without having to carry a magnet around in your pocket.