Auto loan question - with a credit score of 580, would a co-signer help?

I deperately need a new car and have no down payment. My credit score is 580 and I have been turned down by the majority of banks, except a few that were thru dealerships. Also, one lender (not a bank) who specialized in bad credit loans offered me a loan of $5k my payments would have been $500/month and the term 12 months! So needless to say I had to turn that down. I have worked hard on getting my credit fixed up, but I've done almost all I can do for the time being and nothing is instantaneous when it comes to my score. Right now I am waiting to hear back from a large dealership who transferred my application to their bad credit loans department and they sounded like it may be promising, but I know that the interest rate and terms may not be great. The car salesman told me that I might consider having a co-signer - but awhile back a loan officer told me that he thought co-signers were more for people who dont have enough credit, not those who have bad credit. By what I have heard, my score of 580 definately qualifies me for a bad credit loan, but I could still qualify for a better loan, but either way it would be high interest rates. What should I do? I have considered waiting, but I need a new car ASAP.