When i got my first auto loan, by myself, I think i was barely making 12k/year.  I was a college student working part time... my interest rate was 15.99%-- BUT, my loan was only for 6k-7k.  This was in 2007, so I'm not sure what's changed since then.  I think you can get approved for a higher interest rate, but I wouldn't expect to get approved for as much as a new car will cost.

In 2008 we purchased an older SUV (It was 6-7 years old i think), I cosigned with my husband, our credit scores were not great, he was probably high 500s, I was low to mid 600s, and we got a 13.99% 

Since then I've gotten great rates, We refinanced the above SUV in 2009 at 4%,  0% on a new vehicle in 2010 (I was working part time for this one too actually, maybe making 13-14k?) , 1.9% on a certified used in 2011 (cosigned with my Husband), 2.49% on an older used truck in 2012 and 1.9% on a new this week.

Just a little insight to my history.  I think you can get approved, but I wouldn't expect a 20k car.