There is no theft proof car. I had a Volvo PV 544 that had the positive lead
to the coil coming off the back and encased in a steel sheath (like flex
conduit) to the ignition switch. I found this to be quite humorous because all
the thief had to do was bring his own coil with 3 ft. of wire on each end and
clip lead it to the battery and the distributor. There are starters that have
circuits in them that need to be coded before they will turn, but a lot of
good that will slow down a thief who can simply push start the car.
One can hide a cut-off switch (maybe disguised as a wiper or lights), and
loose the wiring in a maze of harness, but again the thief only needs some
wire from a 12 volt source (that he might provide himself) to the coil and
fuel pump, strip the wire and away he goes. Remember he's not tracing down
your wiring... he's bypassing it!
The most effective deterrent would likely lock the drivetrain, but even there
if they have a fork lift your car is still gone in seconds. Perhaps the best
deterrent to having your car stolen (over some distance) would be to slip a
sheet of blackened cardboard in front of the radiator. :-)
Oh, I just though of something else. Have a smoke canister hidden on the car
that will activate if a switch or something is not thrown after the ignition
is activated. That way it is not what they expect to find (to steal you car),
but what they don't.