* The OnStar device is owned by the motorist, not the police.
* The device is installed on every car. I specifically do not think it's OK for the police to dragnet every car.
* The device is by design difficult, in the long run impossible, to remove; it's built into the car.
You seem fixed on the actual information the police are gaining: whether from OnStar or a portable telemetry device, they get a real time feed of where the car is going. Well, they get the same real-time feed from tailing the car. Tails do not require warrants.
I think that they should.
Slapping a $200 GPS tracker on a car is a hell of a lot cheaper than paying for someone to tail you.
As has been said over and over and over by everyone literally everyone here including me, they clearly shouldn't have access to everyone's telemetry data by default. But $200 (and be honest it's the FBI they're probably paying $5000, but, sure, $200) seems fine to me. "If you are willing to put $200 on the line to do it, you should be able to tail someone from a computer screen instead of an unmarked car". Yes, that works for me.
Wait, what? Last time I looked, there were about 6 million vehicles with OnStar. It's certainly on a lot of new cars, maybe even most, but all? In any case, there's a good argument that reading off the GPS data from such a device wirelessly is less instrusive than physically attaching a device to the vehicle.
I just don't agree that sticking a transponder on my car --- that I'm free to remove if I see it --- is really all that intrusive. It seems less intrusive than them staring through the windows of my car.