Before I begin, I need to inform you that I am a Finance Manager at a Mercedes Benz store.
Having said that, most of the people that have answered so far would have been correct up until about 4 months ago.
With the economic crisis in the housing market, and the economy inching closer to a recession everyday (if we're not already there - but that's another topic), banks are losing money left and right. Consumers are holding off on big purchases, such as; cars, boats, houses, etc. These are all items that banks typically finance since most American's are unable to pay cash for them.
Banks, therefore, are starting to lower rates. They have been extending much lower rates to car dealerships because dealerships typically send a lot of business (and money) their way. Many dealerships also have agreements with the larger national banks that prevent the bank from offering a lower rate to the customer than what the dealership can.
That being said, the dealership can now beat out the banks more often than not. You will have to negotiate the rate with the dealership, they are trying to make a profit, too (and you need to take into account that the dealerships are hurting for business in this poor economy). Many times, not all of the time, but many times, the dealership will add two points or more to your rate. This is negotiable! Do not be afraid to ask for a lower rate.
It would be in your best interest to check with your bank, the one that you do regular banking with, to see what kind of loan you can get from them. Again, you can negotiate with them as well. Let them know that you are thinking about just financing through the dealership, make them bend.
All of the above goes out the window if one of the following two scenarios occurs:
a) If you're in the military, or retired from the military, or if you work for the Federal government and can go through one of the government's credit unions, they will offer VERY low rates (i.e. USAA).
b) If the manufacturer is offering special financing, banks and credit unions will RARELY be able to match or beat. Something like 2.9% financing for 48 months, for example, is going to be very hard for a credit union or bank to beat.
I certainly hope this sheds some light on the financing side to buying a car.