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Battery life would be a big con, unless you can hard-wire it into place. GPS signal reception may also be an issue. But other than that, the "Find My Phone" functionality is baked in... And I suspect you can find a number of other tools that would allow tracking.

C
 
So hard wire an old phone to power, throw it in your glove box, and pay for a phone line for it. There's lots of mileage tracking apps; you just need to find one that works for you. Use the "Find My Phone" if your car goes missing, I guess... Personally, it seems like more trouble than it's worth, but I've never had a car stolen. But that's what my insurance is for...

C
 
I have only had my car towed. (illegally) once before but still be great if my car is not where i left it to track it down and find it.
 
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