I'm sorry, but auto manufacturers have a point. Not anyone is competent to reprogram embedded car software. Unfortunately, bugs can be deadly. How would you feel about, say, 10% of the cars on the road running custom software by the next door kid?
People have been tinkering and modifying their cars ever since the first Model T rolled off the assembly line, yet surprisingly civilization has survived. Or are you one of those people that thinks because it's "on a computer" it is scary and dangerous? If the kid next door starts trying to reprogram his ECU without knowing what he is doing he is more likely to brick the damn thing than anything else so he won't be going anywhere near a freeway after that unless he pushes it there.
Cars are killing much more people than guns. In fact, I would go further than the auto manufacturers. Nobody should be allowed to drive a car. How many people on the roads shouldn't have even a driver's license at all? A lot. And you are ready to see those people hacking their own car software? No way!
Oh blow it out your ass, the percentage of the driving population that is interested in modding their own vehicles is rather low, and a good portion of that crowd is more about the spoilers and ground effects than anything. What you are doing using jumped up hyperbole to try and scare people about a non existent threat. I do not support limiting people from modifying something they legally own, however if there ever came some legislation barring you from going outside I would back it 100% because I am convinced you are scared of your own shadow.