It's kinda terrifying that the people making fast, heavy lumps of metal with computerised control systems don't already routinely isolate those control systems from any other computerised technologies in the vehicle, particularly any that can interact remotely. They shouldn't need to be publicly admonished about the dangers of these situations. Don't these organisations employ actual engineers any more?
But given that it does seem to be necessary to make a public display of this -- which presumably removes any plausible deniability if the auto makers do get sued after an accident later, so I can believe it will at least get their attention -- I'm glad it seems to be a responsible group with the right motivations who are starting the ball rolling. If it were just a bunch of lawyers or insurers, the general public could write the campaign off as the signatories just looking out for their own interests.
Problem #1; you can't isolate those systems, in the context of the reason for why they exist.
So, let's look at OnStar, or Hyundai's Bluelink. These are systems that connect to larger infrastructure over public or semi-public communications channels (i.e., cellular) for a variety of purposes. Such purposes include being able to start your car remotely, notify authorities of an accident even if you are incapacitated and unable to call for help (especially in that case, actually) and recover your car in case of theft. All three of those functions inherently require access to engine functions (in a read-write sort of way), GPS, and/or OBD-II data. And you can make a strong argument that many of these things are beneficial from a safety perspective as well. But you can't have them if you isolate the control systems from any other computerized technologies...you absolutely cannot.
On the flip side, you *could* isolate the systems that manage our financial accounts...banking, stocks, pensions...from any other computerized systems. But then you'd lose online banking, bill pay, ability to trade in stocks and other financial instruments without going into an office, etc. But that industry has figured out how to connect things together without the world coming to an end, despite the tremendous opportunity and motivation it provides for criminals. The car industry can figure this out too. I dare say it's easier to figure out how to develop a reference architecture based on the CAN II that is secure than it is to secure all the various interconnections of the financial industry. And it also bears mentioning that once upon a time, the financial industry got egg on their face too for security problems. This is the normal evolutionary process.