I think the question you're asking is more about keyword targeting than about keyword density.
If you think like your own target visitor, when she sits down to look for a dealer who sells the make of car she's interested in, does she type in "Chrysler Jeep Dodge Ram dealer"? Of course not. She's looking for a Chrysler, so she types in Chrysler dealer. Or even more likely: "Chrysler dealer in MyTown. There are many keyword research tools (including the one here at SEOMoz ) that will tell you the other variations of the words your target visitors are likely to use and how much competition there is for those terms.
The other thing to keep in mind is that you don't target keywords to a website you target them for individual pages. Your "website" doesn't chow up for the term Chrysler dealer, it's a particular page that gets listed.
So to answer your specific question, you have multiple, related keyphrases to target for. So you need to build multiple pages, each one (and a few supporting pages) targeted to one of the phrases you identified.
So while your home page might talk about being a Chrysler Jeep Dodge Ram dealer, you should have another really strong page or section that talks all about why you're the best Chrysler Dealer in MyTown. And another page or section that specifically talks about how you're a Jeep dealer, etc. You are essentially building "mini-home-pages" or what are also know as Landing Pages for each of you main terms
A page has the best chance of ranking well if it's clearly focused on one or two keyphrases and their closely-related variations. Then you use the site architecture (how you set up the page hierarchy and links between pages) to help the search engines understand which are the most important pages and which are the supporting pages.
This is obviously just a short introduction to keyword targeting and research, but hopefully it gets you started?
Paul
{Edited to add: To answer your very specific question - Chrysler Jeep Dodge Ram dealer is one keyword. If you want to target Chrysler dealer, you MUST use that exact phrase, not just a long phrase that happens to include the target words somewhere within it. For much less competitive terms, sometimes just having them somewhere on the page (even though not together in a specific phrase) can be enough, but if you're targeting a phrase, you must use that exact phrase at least some of the time.}