Every car hack I have ever seen has depended on either 1) Physical access to the car, that is sticking a new device into the car's network. That is time consuming and not at all inconspicuous. Defending against such attacks would be a good thing, but if you want to sabotage a car and have the opportunity to open it up and tinker with the electronics, attacking the computers is probably the hard way to do it. 2) The manufacturer not paying any attention at all to security. Safety-critical components should not be talking to non-safety-critical components at the very least. This is understandable, because they have only recently started needing to. They will get better as the need becomes obvious. Security is something car manufacturers need to start paying attention to, but car exploits aren't something to panic over. Not yet, at least.
I think the gold standard so far is the jeep hack, wherein the most malicious thing they were able to do was put the tranny in neutral. That said, I'd be curious to hear your thoughts on Michael Hastings. My personal opinion is that the only people who might care enough to break into a Mercedes and plug something nasty into the OBDII port are the CIA, and the C250 he was driving does have a drive-by-wire throttle.
I believe that could be done, and it would not surprise me in the slightest if someone at one of the TLAs had built canned something nasties for a variety of cars, but I find him just getting in a car crash much more plausible than the CIA deciding to assassinate a Buzzfeed writer.