i can slash somebody's tires or mess around with the brakes, hell i can brick the loving engine if i had physical access to the car and a few tools. the problem that you think is there isn't relegated to just automated cars.
There are lots of models of cars out there that have gone completely electronic, that is, software controls the steering, brakes, accel, etc. I believe there have been several ted talks (not sure if many people know about them, they're pretty cool) talking about how every single one they've come across they've been able to hack into and remotely control the car
And the reason that's the case is because there's absolutely no way to stop an attacker that has physical access to said machine, from tampering with it and putting a remote control unit on it.
That's all I'm saying.