Not that I know of on the Mercedes, other than the teleaid theft recovery.
But on the Blackberrys with GPS (AT&T Curve, and some others) you can download a free software program (blackberrytracker.com) that does provide online tracking with updates every 30 seconds or so. It does require a unlimited data plan due to the transmitting of the location.
all you do is: 1.) Install the program and register(free), 2.) Give, or Hide the blackberry in the car (preferably where it can be continuously charged.) 3.) Log in to the website from your home or work pc; and there you go instant tracking.
It may not work on the verizon blackberrys (locked GPS) but I works perfectly on my AT&T.
You have to inform your kids too about the tracking you are doing. Privacy should be important in the US too, this is fine when agreed between parties. There are similar operator services too, these always ask the permission from the located party first.
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