when the door, hood, etc is open the switch contacts are closed therefor completing a ground circuit. when you unplug the switches it creates a open in the circuit and the time control unit/security unit thinks the doors are closed. i cant remember for sure if there are 2 separate units or not, but a look at the online 300 service manual would tell you.
......And just as they're about to catch me.....Twin......Turbos......Kick.......In!