Umm...I'm SURE you thought of this, but here is a reminder. You could always just buy a GPS-tracking solution from Viper/DEI.
Of course, there is a monthly/annual fee, but it is less than what you would pay for an extra cell phone line. The advantage with this system is that it is completely independent of a carputer so all the usual problems won't go along with it.
Viper GPS Tracking Unit