As corporations start to paywall features in cars, features which are clearly physically present but must be subscribed to in order to activate, I expect to see a boom in the number of mechanics educating themselves on the new systems and charging fees for "car feature unlock" services. It's been happening with mobile phones locked to specific networks for decades, and car mechanics can recognise a good opportunity to make some quick cash as readily as a mobile phone salesman can.
In the short term, car manufacturers can reserve access to the software for their own dealerships, but that only delays the inevitable, because older cars will inevitably have to be maintained by smaller, independent garages, which tend to be cheaper than the main dealerships. By the time the cars are 5 years old, maximum, the independent garages will need to be able to perform repairs and updates to the onboard computers, and from there it's only a matter of time before some clever, innovative mechanic figures out how to remove blocks on features and bypass subscriptions.
And that's assuming that as cars become increasingly computerised, nobody figures out FOSS operating systems and software for them.