I actually do finance at a large dealer, since you're under 620 you'll go to sub-prime or "secondary" bank, so the rates will be high, DO NOT continue going to well-known banks and credit unions and having them pull your credit, the inquiries will kill your score even more, some banks will shop it around and ring up tons of inquiries. The well-known banks are all prime lenders, Bof A, chase, wells fargo, almost all credit unions, you'll have to allow the dealership to find you a secondary bank, careful because most dealerships are allowed to make 2 points of rate on top of the banks "buy rate" so work the finance manager by telling him you found lower rates elsewhere or something of that nature.
The key when getting approved for a car loan is Loan-to-Value, ultimately you'll have to buy used since they can be marked down further, do all your shopping on the internet, the dealers discount the cars more online for competitions sake. What the banks will finance you on, is something 35,000 miles or less that is 2003 or newer, since the vehicle is the colateral the bank wants something that, if you you default on the payments and they have to reposess they can auction and get some of their money back. The biggest key is that your loaned mount is significantly less than its retail book value. Also, they'll probably give you $14,000 max to spend. The banks make you crawl before you walk, so they won't approve you for alot.
Example:
2005 Chevy Impala
27,000 miles
retail book value $15,700
purchase price $13,700
+inequity carry over from trade-in $1300
=amount financed before TTL: $15,000
Loan-to-Value: 95.5%
obviously money down makes the loaned amount less and the loan-to-value ratio lower, so try to put as much money down as you can, the odds of getting approved are much much higher around 80-85%, just like buying a house. and remember Loan-to-value doesnt have to include TTL, you will have to pay TTL but the banks look at L-to-V before TTL just so you can do you math at the dealer while looking at cars.
So stop going to prime lenders and getting your credit pulled, the dealer probably has a secondary finance department that can get you approved with the right Loan-to-Value ratio with your credit, and yeah, I know it sucks to have to trust the dealer because they're going to make money on you but hey they can only make 2 points of interest rate so if its an inexpensive vehicle it might only be a few hundred bucks but thats the cost of using their services and atleast you'll get a newer car potentially with some warranty still left on it. Just dont buy any extended warranties or service contracts, those are where they really make money. Definitly get gap insurance incase something happens but don't pay more than $500 for it, the dealers cost is usually only about $350 for it.
READ THE PURCHASE/BUYERS ORDER CAREFULLY BEFORE SIGNING IT AND THE CONTRACT TOO, make sure all the blanks are filled in and read all the lines, they can sneak expensive non-factory warranties in the financing and tell you its included with the payments on your new car, but they have to disclose it to you on the purchase/buyers order to have the banks fund it, alot of bad credit people are so excited they just start signing away and don't read what they're paying for. Expect interest rates in the teen's for your score, so on a $14000 loan expect to be paying in the high 300's for 60 months (you wont qualify for 72, so don't bother)
If you do all this correctly you'll get a good deal, and the dealer may still make a small profit but they should because they have a business to run and they run alot of paperwork to sell cars, but you wont be getting ripped off for thousands. Pay on time for a year or two and you'll be able to trade it in for a new car and prime interest rates, it'll cost a little more now but its worth it, a well paid car loan will do wonders for your credit.
