No, there are quite a number of computers that are isolated from wireless and internet for various reasons. The USAF's secret space shuttle was kept a secret for quite a long time because articles related to it were never placed on DARPAnet. Then there was the stealth boat, the one you see them use in the Bond film, "Tomorrow Never Dies" called the "Stealth Shadow" and nobody knew that thing existed until it sailed out into the harbor, even people that lived next door to where it was being constructed.

One of the rare few movies to get hacking right was "Live Free or Die Hard". The hacker character in that explained that many key infrastructural systems for power generation and the like are completely offline. That's why they had to kill several guards and go there directly.

In an autonomous vehicle, it's highly likely that the core systems are going to be firmware and that they only way to access them would be through a custom proprietary port like a USB but only available from the car manufacturer. The actual internet in your car would be handled via a completely separate computer. I mean, think about it, why would you want a car that needed the internet to drive? If there was a power outage every car in the area would immediately go dead.

There are some thing you just don't bring online. For example, when I was playing Shadowrun and playing a Street Samurai I had my assault rifle which was chipped and had an awesome firewall thanks to my "hacker" ally but I still carried a good old fashioned mechanical .454 Casull Magnum revolver for those special occasions.