"Because infotainment systems processed DAB data to display text and pictures on car dashboard screens, he said, an attacker could send code that would let them take over the system.
Once an infotainment system had been compromised, he said, an attacker could potentially use it as a way to control more critical systems, including steering and braking."
Normally it's not that easy. Sure, the car stereo sits on a can bus with nice information (ACC, backing signals to turn on the back camera, speed information so the volume can be automatically adjusted, etc). But it's not on the vital CAN bus (at least not on most cars).
But yes, it's an entrance point. So is the 3g/wifi receiver in the stereo, or the bluetooth connection to the handsfree that it can do.
But you would have to:
1. crack an entrance point to the stereo (any of the above)
2. control the stereo CAN transmitter (if it has one)
3. using that CAN to crack an entrance point to another system that talks to a vital CAN bus
4. control that system enough to transmit CAN on the vital bus
5. and then use this system to send bad messages to brakes or steering
and all cars use different firmware with different security holes and different CPUs.
But with enough research you could probably crack a specific vulnerable car model.
Cracking modern airplanes seems easier, actually.