Building a vehicle tracking device
These are already being used in fleet vehicles and even walmart sells them.  Don't know the actual device name, tracking phones or something similar.  They can be used for example to track where your hostile 17 year old really drives off to when they say their only going to the movies.  Best bet for low cost would be to buy second hand devices, likely loads of them available by now.  

The received caller ID would tell you the specific device (and you then look up the vehicle ID that has that phone number - you could use an automated lookup device of course).  The setup of the tracking function gives you the phone's location, and I assume the GSP Lat/Long is also an option, (so from that you can track down your teen and read him/her the riot act..... and how many days they will be grounded....)