Can you wait a bit before you buy a vehicle? Check out the rebuilding forum here to see what you can do to increasse your score to the mid 600s. Some people have successfully increased their score rapidly. If you get into the mid 600s you should be able to get a good loan, nowhere near the 15-18% that has been quoted. There may be some quick things you can do to boost your score. Read the rebuilding forum to see what you need to do.
Also, before you go to the dealer, pull your 3B report and scores here so you know what your auto enhanced score will be: http://www.myfico.com/Products/FICO-Score-3-Report-View/ Not all dealers use auto enhanced scores, but many do so it is worth it to know your score.
Lastly, do you have a credit union you can get a pre-approval from before you go? Then you will have a much better chance of getting a decent rate, even in the low 600s.
