1. Get pre-approved for the best loan you can from a local credit union
2. Go to dealer and negotiate best price on car
3. When you get a price hammered out, tell them you are finaced already at X.XX%. Ask the dealer if they can beat it. If they can, good. If they can't, oh well. You'd be surprised at the rates they can get.
I'd also consider a pre-owned car from a brand like Nissan that frequently offer very competitive rates on their used vehicles.
Bottom line is to negotiate the sale price and the interest rate independent of each other.