Outside of going in and getting preapproved for a credit union loan, believe it or not, most reputable dealers have relationships with banks, which can actually get you a better loan rate, than you could walk into same bank and get. The reason is based on volume and relationship. They also have the ability to negotiate for you, based on same banking relationship. Banks sell money to dealerships at a lower rate than to individuals. That pretty much goes with any product you buy..wholesale, vs. retail.
Dealers have "buy rates", which are the wholesale rate given to the dealer, based on loan structure, terms and, of course the credit rating of the buyer. ie., xyz bank might have a published rate of 7.9% for a new car, 72 month loan, but the dealer might be able to buy that money for 6.75%.
Going dealer direct also cuts down on the time you must shop for a loan, since they are normally plugged into as many as 30 + lenders, know the criteria, and in most cases have access to electronic instant approval. 🙂 As an example, I shop up to 5 different credit unions, for the best terms and rate for a customer, all within 5 minutes of when I press the submit button. 😉