Ronnie, you can PM me if you want more detailed info on your situation. I've done literally thousands of "non-prime" loans, and I'll give you the skinny, no strings.
As for the co-buyer, most bad credit sitch's don't get going with a co-buyer, unless the sitch is marginal, and the co-buyer is super strong.
As for the higher rates, 12-18% is not exactly loan sharking, dept store credit cards are double that.
Interest rates are just like insurance payments, but insurance on the bank's money. If you had a fire in your house, you'll pay a higher premium for the next few years until you've re-established your recod. Your student loan was the fire, now you have to prove you can pay your obligations.
The key to high rate finance is not be in in for 5 years. And DON'T GET A NEW CAR!!. The higher rates means your paydown rate is slow, and in 2 years you will be in a serious negative equity situation. Buy a 3 or 4 year old that has had most of the depreciation already occur.
2 years into the new loan, you will have re-established solid high installment credit, and with job and resident stability you may be an ideal canditate to qualify for prime rate programs.