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Doing this would be a terrible idea but if you have to help your family out, get a cheaper car with payments that you can afford to make if they don't pay. Make sure they give you the money each month to pay the bill and don't put the car in their name so you can repo and sell it in case they fail to pay you.
Even though this is so old and you probably already made a decision, I just wanted to give my $0.02:
ABSOLUTELY NO!!!
I have had a lot of credit problems in my life and I have worked very hard recently to rebound. If I were someone else, I would never have cosigned something for me. Your parents are unable to get their own financing for a reason: they haven't paid their bills. There is no reason for you to risk your own credit because they might have changed that pattern now. They can take the time to rebuild their credit to a point where they can get their own financing. In the mean time, you protect yourself and don't take any chances. Anything bad that might happen would haunt you for 7+ years. It is absolutely not worth it.