The title of this post is "solving... " but there weren't a lot of solutions -- or any -- given. The answer it seems is that we secure inter-car communications the same inefficient and relatively easily hacked way that we secure the internet. Doesn't really give me a lot of hope.
Well, I do think this is a solution. The author's point, as I interpreted it, is that automakers will not be exempted from following the well-proven, well-understood authentication process. Cumbersome it may be, there will be no shortcuts.