I agree with bd77, you should have paid listings.
At least have it so they can upgrade their free listing to a paid one where their ad/link will appear at the top...
You could create a mailout to each of the dealers, tell them about the website and offer upgraded listings. Basic (free) listings would just be a text listing with their address. Upgrades could include a link to their website or email address in their listings.
I created a similiar website for someone, but it was for Florida Realtors. It started with ALL realtors being listed for free and the owner of the site is working on contacting and upgrading all of them.
http://www.FloridaRealtorList.com