If you are absolutely certain that the switch is not connected to anything on the car then you can treat it like any other pushbutton switch connected to an Arduino
Connect one side of the switch to an Arduino pin and use INPUT_PULLUP in pinMode() to activate the internal pullup resistor to keep the pin in a known state. Connect the other side of the switch to GND so that the pin is taken to GND when closed. With that done digitalRead() will return a state of LOW when the switch is closed
It is worth noting that a car is a very hostile environment for electronic components of all kinds