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Since you have CC's. And soon no installment loans. Thats the route you'll want to take for the Credit Mix. Even if you want to keep the car. Try a CU for a SSL or personal loan. Or even the bank that financed you. Its cards and 1 loan that does the trick. Not another card. Good Luck!
I agree if it is in your budget/sights then I would opt for the Car as well but it will still affect your age as you will be adding a new account regardless but with it being in seperate sections having Cards and a Car/Personal loan will look better on your behalf.