automated flying cars ... would be ready-made tools of political assassination
When you see someone use their turn signal, you make decisions based on that information, but you don't bet your car or your life on it - you give yourself enough leeway so that if the information is wrong you do not get into an accident.
Yes, automated cars will share information, but they will treat all incoming information as suspect until or unless the information is certified.
Cars will be autonomous in the same way drivers are. They will talk with each other, but they will still drive as though everyone external to them is feeding false information, and treat all incoming information as hints and suggestions rather than actionable policy.
As far as trusting the code in the car you're riding in, it's really little different than trusting that your automotive manufacturer designed the brakes correctly. The modules that control the car will be completely proprietary and encrypted from the manufacturer, and any change in code will be detectable via encryption signature - the car won't run if the code is altered.
Besides which, we've already passed the point of no return. The high end cars that auto park themselves also have braking control, steering control, and the throttle is electronic. The computer can control all three variables (throttle, steering, braking).
Further, they receive wireless data from both the keyfobs and the tire pressure/temperature sensors. This data goes right onto the vehicle data bus.
So, in theory, if they have errors in their code, or exploitable weaknesses, then what you're worried about is within the realm of possibility now. No need to wait for a fully automated car.