I don't know exactly what Google has up their sleeve, but by watching how our SEO performance changes and speaking with some people who know it better than me I have a theory:
Google's mission is the provide the searcher with the most relevant and timely results. Within that mission, they want to give their customers a way to get to the end result - no middlemen. In the case of new vehicles in the automotive industry, this would mean the end result is a car dealership's website or the actual manufacturer's page. Used vehicles get a little tricky because there are independent sellers and private sellers that are just as relevant and timely as a franchised dealership. But, Google is still trying to get the person past the middleman. So, if the middleman is Cars.com or Autotrader.com then they will either benefit (in Google searches) by a market of non-SEO-knowledged dealerships or will have to do more with PPC advertising in order to breach Google's goals.
As for Google Base, I think it was an experiment by Google to see if they could be the middleman themselves, but then realized it did not jive with their mission. That, or they realized there is a better revenue stream from somewhere else, so they don't do much with Google Base anymore.
These are just my theories - I have nothing to back them up.