In this specific example: are car dealers not participating because they don't trust the central party (case for blockchain) or because (lazy|not in their interest|inconvenient|...)? If the latter, then blockchain is not solving the actual problem.
Right now, selling a car with an official maintenance book is always a benefit.
The real benefit is of course on the buyer side, who wants to verify what he is buying. So a car with an "official" log would be more valuable than without. When this is the case, it of course is better for the seller to have such a log.
I've been in this space for a short while, and I can definitely see it working.
When you buy a car right now, you expect it to come with at least a maintenance log. So that part is not an unknown.
Everything I mention already exists, it's just that it would be way easier and more complete when it would be handled by a 'standard protocol', ie a blockchain.
The same can be said for supply chain applications. But the second hand car industry has even more trust issues. Selling a car for $8000 and getting around $2000 extra because you can prove it has a proper history is not fantasy.