The only guarantee you have when it comes to theives is theft insurance.
Gone are the days of the simpleton smashing a window and hot wiring the car for a speedy getaway. These guys literally do it for a living, if they want it they'll take it, pure and simple. No amount of alarms, switches, disconnected circuits or anti thrift equipment will stop them.
A friends wrx got stolen from a train station in broad daylight, it was alarmed with immobilizers and a steering wheel lock, police closed the case a few short days in due to lack of evidence and leads. Cc tv at the station wasn't even useful enough to id anyone or anything.
Insure it, you at least get a small some of money back, because you'll never see the car again.