You bring up a number of good ideas. But remember it is not simple. Autonomous cars are electronic ecosystems unto themselves, with processors, storage, interfaces, networks, and users. The same issue we deal with daily to protect servers, data centers, networks, the Interent, PC's and phones apply to autonomous vehicles. There are no simple answers or solutions (else the rest of computing would be secure by now).

Smart minds are working the problems, both on the good as well as attacker side.