Good ideas. I'm wondering if this multi sensor would work:
http://aeotec.com/z-wave-sensorThe most important will be the vibration sensor coupled with the motion sensor. I think it's better than the "Vision Z-Wave Shock" vibration sensor which is solely a vibration sensor and requires being attached to the glass, which would not be practical and a little conspicuous.
I'm thinking the Aeotec multi sensor could be installed somewhere down in the console or under the drivers seat. That way the motion sensor won't be set off any time someone walks by the windows but will be set off if they actually enter the car. Also, since it couples as vibration sensor, hopefully it's sensitive enough to notice/alert when somebody is breaking the windows and/or entering the car.
I have no idea how it will behave constantly leaving and returning to the network. Hopefully I'll buy around Black Friday/Cyber Monday and then try it out. If anyone reading this has already tried (or tries) this particular Aeotec multi-sensor, feel free to share your experience.