Currently to power this I am planning to attach the 12v sensors directly to the battery. Ideally I will use a relay or optoisolator to power them down when I disarm the alarm. I plan to power the arduino circuit by dismantling a car cigarette to usb power adapter.

Given that cars are notoriously noisy environments for electronics, I'm hoping that the alarm sensor is designed to filter out any noise generated by the Jeep. I am also hoping that the cigarette to usb adapter takes care of the noise. If noise turns out to be a problem I will look into adding de-coupling circuits.  I found this article informative on the subject.