Oh man, what if he panics and calls 15!
Its very difficult for you to handle, because of the sensitivity of this relationship. I tell you what you do ...... pray that he gets his own car, and i do that you get employed.
Some people never understand, regardless of how much you teach them.
But its your father's sister's home. Just let your parents know that you care so much about your car but you care more about your family and parents, and only that's why you would always be in debt no matter what. I hope your parents would take care of it then, and if they don't or can't, just try to put up with it. It might not be a very wishful advice, but you have to share a lot in life. Try to settle matter, and don't plan to settle the score.
-> Never ask them to keep it clean, this would hurt them really bad. This would make you appear shameless without any obvious gain.