Because of the credit crunch and financial situation in the US, the lending guidelines have changed for most car dealers. Before, people with an income of $1,500 a month or more could get approved for a car loan.  The situation has since changed.

These days, car dealers will typically consider potential buyers who make at least $2,000 a month. If you are thinking about purchasing a car with auto financing and you make less than this amount, get a part time job, or wait until you get another raise before you apply. Applying prematurely and getting denied will only add a credit inquiry to your credit file, lowering your overall credit score (which would make it harder for you to qualify in the future).

If you are self-employed, be sure to file tax returns annually showing as much income as possible.  Some self-employed people will do whatever they can to hide income, especially cash payments such as tips, from the IRS.  When time comes for them to apply for a car loan, they soon realize what a grave mistake they made since tax returns effectively serve as your proof of income when you are self-employed and cannot provide pay stubs to a car dealer. (not to mention that hiding money from IRS is illegal)