Trying to get an auto loan?

Ive got a 2003 Kia Optima and I'm thinking about trading it in for a little bit newer Jeep Grand Cherokee. The blue book value on my current vehicle is 3800 and the one I'm wanting is about 18000. I recently though applied for a personal loan for 5000 from my bank but my length of credit history is too short. I dont have bad credit. My credit score is 714 and I have 6 hard inquiries. Some that are a few years old and some that are recent. Would my best option be to have a co-signer because I'm afraid that if I try to apply for an auto loan I'll get turned down again?