Submitting an application for an auto loan initiates a 14-45 day "shopping period", during which all inquiries of the same type are counted as one by the credit bureaus. This means that if you're shopping around for rates, you don't have to worry about continually affecting your credit.
For most customers, the first credit inquiry will result in a temporary five-point (or less) reduction to their credit scores - a generally inconsequential number that typically goes away after 3-6 months.