Gone are the days of $300 car alarm systems ore expensive Lo-Jack surveillance systems. Well, not quite in some cases. Even if you already have a factory-installed car alarm or some kind of mobile tracking security system, you can create a pretty inexpensive one
using a cheap-o mobile phone and a standard car charger for your selected mobile model. You'll need some elbow grease, tape and maybe some wire as well.
Looking at the video below, this system is so simple it's ingenious. The premise works like this: you hotwire a mobile phone charger directly to your car's battery (thereby bypassing any fuse block issues), and connect it to your chosen mobile phone This can be a spare unit or a cheaper prepaid unit. This ensures the mobile phone has power all the time -- hence, it is "callable" all the time, and local cell towers are in contact with it constantly.
Now, if your vehicle is stolen, you can give the authorities the IMEI number from that in-car 'hidden' handset, which can then be tracked using cell tower triangulation or GPS, and hopefully recovered. You'll of course need some kind of airtime subscription to ensure that phone is operable at all times.