Since the 30 wire (always battery voltage) feeds all accessory and run conditions, it may be possible to monitor voltage to an "always on" component, such as the clock or parking lights. Jump wire a low wattage bulb and socket, such as a test light into one of those circuits and place it where you can see it safely while driving. If it stays lighted when the engine dies, the problem is most likely in the ignition switch. The same test can be used on the 15 wire from the switch after you know that the 30 circuit is always live.