Well first, if your car has PATS, then it's tough for people to just hotwire it, you would need a chipped key to start it. And if you're worried about people somehow getting your key then trying to start your car, just wire a kill switch into the fuel pump line, you can tap the wire at the inertia switch in the driver footwell (i don't believe that there would be too many adverse effects from this; but someone will probably correct me if needed
). Another thing, i would think that any dedicated thief could just yank that keypad out and mess with it that way. If you really feel the need, i believe that a hidden kill switch would be better; but i guess that thing is alright if you want it for the novelty or whatever.