Consider a GPS-based system like GeoTracer instead (
http://www.geotracer.net/GeoTracer).
It uses GPS to locate the car and analog cellular service to contact it. Specially equiped police vehicles are not required. Anyone with the right PIN number and a web browser can find out where their vehicle is, what direction it is going, and how fast it is going at any moment.
It would still be a problem if you put it in a shipping crate, though. It would still report the last known GPS coordinates if it still had cellular service inside the crate, but if they moved the crate you are SOL.