GPRS Tracking

I think you are looking at two different parts of the issue. GPS will give you car location (GPRS may assist but will not be as accurate) and that location information will be transmitted by GPRS to the phone network. Once there how you read the signal is relatively flexible. There are a number of commercial systems available that do exactly that.

 

Garmin GTU 10 GPS Tracker is impressive. I've used it. You can have an app open on your laptop or on my iPhone. With that app you can see where the tracker is located. You can set up these things called GeoFences. Let's say you set up a GeoFence around your neighborhood. The tracker will go to sleep while it is inside that GeoFence, conserving batteries. When it leaves the GeoFence, it will wake up, an email or text can be sent to you that it has exited the GeoFence. You can set up a number of these geofences. So you can be informed that the target car has left home, entered another geofence (area), etc.

The problem I found with the Garmin is battery life. It is supposed to last for days on a charge, and I found that often it did, but I found it could die in as little as a day. If there is anyway to connect it to power in your target car, you would be golden.
http://www.amazon.com/Garmin-GTU-GPS-Tracking-Unit/dp/B004HFRA7A/ref=sr_1_1?ie=UTF8&qid=1313533022&sr=8-1
Very impressive though, for a couple hundred dollars
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Another idea is FindMyiPhone or a similar app on a phone that is in the target car. Or Google Lattitude. Both of these allow you to see where the target phone is. A buddy lost his iPhone on a ski slope and we were able to locate it using the FindMyiPhoneApp.
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Then there are spyware programs you can install on the phone that will allow you to GPS track it, but those are for sure illegal. I don't know about the legality of the garmin unit or the FindMyiPhone/Google Lattitude programs. There was a recent court case about that, but I don't have it handy.