17 cars -- I assume you, yourself need one, and your mom needs one. So 15 cars are just sitting around depreciating. You say most are worth 4-5K. So if you sold 15 cars, you'd have 60-75K. Plenty to buy her ONE nice, new car and make her happy. But the "price" of that one nice, new car is to sell the others.
I do believe you're dealing with mental illness, and that's tough because she THINKS she's being reasonable. Since she used to be a car dealer, she must understand the concept of depreciation. Can you come at it from the point of, "This car is worth 5k today, but if we let it sit another year it'll drop to only 4K?
Things that might help:
- The Hoarders show really might be a good idea. Yes, they do provide money for follow-up services. Some people take advantage of them, and others don't. In most cases, I'd agree with you that lack of follow up = you might as well have done nothing, but your case seems different: Most of those people can go buy more clothes, more food, more junk without anyone's help, but since your mom hoards cars (and she'd need your help to obtain more cars), she literally could not continue to accumulate.
- You say she sees this behavior as "normal" because her friends do it. Could you widen her circle of friends? Look around for senior citizens groups. Our church has an active senior citizens group that gets together for pot-luck lunches, does service projects, takes bus trips up to the mountains to see the fall leaves, etc. We also get a flyer once a month from our city that offers various programs: Kids' tennis lessons, etc. Lots of those offerings are for senior citizens: Walking for exercise groups, teach kids to knit groups, learn digital photography sessions. If you could get her involved with some other people, perhaps she'd see that other ways to live exist.
- Take her off your credit cards, and instead start giving her a cash allowance -- and if it runs out, don't bale her out. Let her have nothing 'til next payday (or whatever date you've agreed upon). Yes, it means you might have to do the grocery shopping, but you're not happy with what's happening now, so changing that detail wouldn't be too bad.
- I'd be concerned about her getting credit cards in her own name, even with limited income.
- Giving her an ultimatum (reduce to two cars -- I'll even help you do it) by the end of the year, or I'm moving out . . . is an okay thing to do, but it will come with its own set of issues. First, you have to be willing to follow through. Are you? Second, she'll be angry, and you'll probably fuss about this for the rest of your life -- few adults like to be told what to do.