I don't know of any affiliate programs specifically geared towards that, but you may want to check out eBay's affiliate program. There are normally a few used cars on the site being sold, and even if your users don't buy a car, they may sign up and buy something else, giving you a nice commission. Their referral cookie lasts for two weeks (I think?), which can lead to multiple affiliate commissions along with a sign up bonus if they're a first time user.
If you have a used car site you may want to also tie in offers for financing, accessories and car insurance. Check Commission Junction too, I think I saw some affiliate offers from some of the used car portal sites.