That's too late.
Even if you can avoid the security/safety issue in your car (which I doubt you will be able to), you will not be able to avoid it in the next place where embedded (computer) systems (security) will raise concerns (tubes, trains, planes, houses, nuclear industry, chemical industry, ... put your favourite risk here ...). It's even possible that the automotive industry is not specifically "in advance" on this topic...
The problem is taking seriously into account computer security. I had hoped in the 90s that maybe this could be done before computing invaded everything. It seems I was wrong.  So now, what do we do to change that state of fact (before even your old no-computer car really gets unusable)?
Switching to Linux may be an improvement.
But note that if I had the choice now, I would switch to OpenBSD. Not because of the technical quality, but because of the design target.
(Unless Linus and other developpers of the kernel clearly upgrade the priority for security of course.)
PS: Another practical idea but intended for cars manufacturers: offer brand new cars to all linux kernel developers. Now. And for BSDs devs too (come on, that business is not *so* in crisis). Let's remember them that was what Digital did 20 years ago to get Linux on its Alpha CPU.
 In the meantime, in my opinion, security only seriously expanded to the gaming industry and to some extent the media/telco. industry. What an irony!