So far, the systems controlled by car computers are burned into
PROM chips supplied by the manufacturer. They don't have the ability
to communicate any further than a socket under the dash, where techs
plug a hand held device that can download data to assist them in finding
causes of performance and/or emissions problems. To introduce any
programming, legit or virus into the system, you have to replace the PROM.

They are invulnerable to viruses because they are so primitive and lack
the IO capabilities of even the earliest PCs.

Wait a few years, though, when car makers are tempted to add lots of new
capabilities to the system. Maybe some day they will be susceptible
to viruses, but (thankfully) not yet.