I foresee two primary ways car "hacking" would likely be used:
1) Mass scamming, in a similar vein to current spam e-mails. It doesn't matter if only one person in a hundred falls for it if you're contacting tens of thousands, you're still making big bucks. This could be pure fake, with "hackers" convincing car owners their cars have been hacked when they really haven't, or it could be real, with something simple like the car refusing to unlock unless they pay X sum of money to Y account within Z time frame. And it could all be easily done using cryptocurrencies to make tracing more difficult. And with digital payments (e.g.: Swish, digital direct bank transfers, crypto, etc) done on your phone becoming more and more common, you wouldn't even need to leave your current location to perform the payment, either.
2) Targeted hacking for the purpose of general terror attacks (e.g.: driving a car into a crowd) or assassination (either hack the car the target's in and drive it off a cliff or into oncoming traffic, or hack a nearby car and use it to run over/into the target). Could also be used as distractions or blockades to make certain crimes easier to carry out (e.g.: the police is going to have a much harder time apprehending bank robbers if civilian cars are suddenly getting hacked and rammed into pursuing police cars, or simply used to create a "blockade" behind the robbers' escape vehicle).
All of this of course assumes that automated cars have their steering in some way connected to wireless systems. If they are kept separate, then a lot of the above will be functionally impossible to carry out without physically getting into the car itself and sabotaging it.