Yelp, Yelp, Yelp, Daniel! Are you blogging about your bad reviews on the dealership blogs and answering the bad reviews?
There is always two sides of a story and the customers are willing to here it.
That would be the first thing I would do to overcome them showing up in the SERPs.
Question?
Why would anybody use other review sites and not build their own?
Who would have control now?
How can they keep you hostage now?
I do believe that reviews are important with Google and only a handful of review sites, but build and take care of your own review site first, than you can reach out to your happy customers and ask them for some help on the others sites.
www.YelpQuirkAutoReviews.com
PS A happy customer will do almost anything for you!