First time car buyer with 50% down and still got a high interest rate?

My gf just turned 18 and has no credit. We bought a car from Carmax and put a down payment of $10,500. The car costs $19,748. The Carmax financing is giving her 14% interest even with the down payment of more than 50%!!!! Besides getting a co-signer, is there any way to get a lower rate? And no, we cant use my credit because I just finished paying off 4 maxed out/over limit cards so my rate would probably be higher....