A remote BT connection hack on the go is near to impossible, the hacker must follow me very close for quite a long period of time to be able connect to my very low range OBD dongle and stay connected long enough to communicate with the computer.
With the right antenna, I'd expect to be able to succeed in this sort of attack if I could stay within three hundred yards or so. It would take a significant effort; best chance of success would probably require a driver for the car, a hacker on a laptop, and a dedicated antenna-aimer.
Hacking cars without physical access can be very easy (for cars that have Bluetooth or Wifi devices with security designed by automotive engineers rather than infosec professionals) to impossible (my car has no such integration). With physical access, anything can be hacked.
Cars that have life-safety equipment such as brakes that are remotely accessible are cars that I for one do not want to own.