The best solution is the simplest: no closed-source systems allowed in cars used on public roads. But I wouldn't hold my breath until everyone gets on board with that.
Or suppose a neighborhood kid is angry at you, have figured out how to hack the system, but haven't yet figured out the difference between "that'll teach them a lesson" and "this might actually kill them".
Or, hypothetically, if system hacks don't require a physical connection, it's wide open for anyone anywhere in the world to replicate something like the file encryption extortion scam[1]: Break into as many cars as you can. Send them a mail saying that you hacked their car. They can take the chance of figuring out what you did on their own, or pay you money to revert it. The scam might work just as well for cars you didn't break into, as long as the owners believe it's a credible threat.
The point isn't necessarily that these scenarios are more likely than in the physical world. The point is that many people have a fair idea how the physical world works, while they have only vague notions about "hacking" in the virtual world. We know that there are new threats, but we don't yet know what they are, so these new threats will be inherently scarier than the threats we already know about. (The devil you know, etc.)
[1]: http://www.techspot.com/news/17678-file-encryption-extortion...
I personally would like to see various "hacks" adjusting the suspension, brakes, spark timings and other things for a better ride in certain conditions (racing, drifting, mountain roads, etc).
