You could use a pull up resistor on each input. the resistor would just connect to +12V and then to your door trigger.
**broken link removed**
R1 is the pull up resistor, s1 is the door switch and the inverter (triangle thingy) would be your circuit.
When S1 is closed, the path of least resistance is to ground, so you would see 0V at your circuit. When S1 is open, there is no path to ground, so the next best path is through R1, which pulls the output up to 12V.
Hope this helps, and welcome to the forum.