I like the contest idea, but, those that have one don't need another, and those that are going to buy one, have already found you. Although a new buyer would sure like a "contest winning price", and perhaps that in itself would generate extra word of mouth. How do you advertise other than the website? If you have regular Radio, TV, newspaper ads, then, sure, have names submitted have a draw one winner contest.
The whole series for this year could have a special paint job, or at least a special decal.
Free anniversay bumper stickers, given in the show room & given with requested literature.