Only the first one is possible. The second one should probably be 'did you fix the car?'. 'Do' and 'have' are both auxiliary verbs in this context, so you need one or the other, not both.

Have you fixed the car is the correct way to ask the question. "Do you have fixed the car" if intended as a question is wrong both in word order and the unnecessary word do. Will you have the car fixed? is a way of asking the question in a future tense mode. And Have you fixed the car? is asking if the car is already fixed, that is past tense. Hope this helps you some.

The second one is wrong. Only "Have you fixed the car?" is correct.