Apparently, most cars now require 3 cables to be connected in order for the engine to start.. you'll need to connect 1. the Earth wire (obviously), 2. the cable that connects to the ON switch, and 3. the ignition cable.. only then will the car start and the car will remain on if the ON and Earth are connected..
So, unless the thief is a real pro like in some Hollywood film and is able to start the car by connecting some wires, a small switch on the ON cable might just work.. most people here use it to turn off the car in the event the engine is too hot and the car refuses to shut down (race/drift cars) I've just installed a small ignition switch for starting the car up, but u'll still need the key to be in ON position for it to work and stay running..
But the most basic thing to preventing a car theft is not parking in a secluded place.. park in a place where people pass by.. use anything that may prolong the time it takes to steal a car (steering lock, brake pedal lock, wheel clamp) because most thieves wont steal a car if it takes more than 10mins to do it.. it just attracts attention..
Just my 2 yen..