Ford XD Kill Switch

I have a 1979 XD fairmont and have put new $2000 mags and tyres on it... i have locking nuts but i know how easy it is to get into and start... im wondering what the easiest way is to put a kill switch to the car.... would it be just simple to set up a switch to break the (-) conection to the ignition... and if so which color wire do i break... im not much of an electrical guru.. an am just looking for something simple??? Help me please