Most people have no idea that their car with telematics features (ie Onstar or similar) is almost constantly reporting their location and a slew of parameters back to the manufacturer.

This is true regardless of whether you have a telematics subscription. The subscription just enables the stuff you can actually use.

Manufacturers are monetizing that information, selling it to anyone who wants to buy it. I know someone that worked for a company using said data to try and enhance weather reporting (the data reported back includes outside air temperature, headlights, windshield wiper status, etc.) They were receiving data from multiple manufacturers.