High level thoughts.
Thought #1). IF ALL ADVERTISING WAS FREE, WE'D USE ALL OF IT (print, mail, billboard, TV, digital, digital classifieds, etc). This means the discussion is about ad spending yield.
Thought #2) "Dealer's are like snowflakes, no 2 are alike". Advertising must match the dealer's profile AND the dealer's profile must match the dealer's ad strategy. For example, a dealer who's inventory is poorly managed (selection & price) may have high ROI from direct mails (and poor ROI in digital classifieds and short tail PPC). Another dealer profile is the big fish in his market. This dealer has a giant inventory and ad spending with a marketing strategy to push shoppers directly to his site. The big fish's strategy is to become an internet shopping destination where he competes with AT and Cars and Gurus. This dealer can justify very short tail keywords with far higher CPCs.
Thought #3). Like Ed said, it's all about balance. It starts with the Dealer's profile 1st. Once you have the dealer's profile (i.e. strengths and weaknesses), THEN you can strategically deploy ad dollars.