I think we all wish you luck. You seem to be the only responsible one involved in this mess and you're trying to claw your way out of it.
IMO, though, you don't seem to gain much by this scheme. By trading in a car & getting wholesale for it, you'll get, at retail, an older and likely, less reliable car in the exchange. The money you save by reducing your principal may just go to more car repairs, so the total cost may not be much different.
I hope that you got something in exchange for the debt you've taken on, or that your parents appreciate the favor you have done by taking on the consumer debt they foolishly incurred.
Good luck.
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