All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
If you drove the car while aware of a problem in its brake system, then you were legally negligent. You should have taken the necessary precautions and fixed the brakes before driving. Therefore, you must pay the Diyah [blood money]. If this young woman was non-Muslim, then her Diyah is half that of the Muslim woman, as we have clarified in Fatwa 83489.
There is some disagreement regarding the obligation of the expiation for killing a Thimmi [a non-Muslim living in a Muslim state], though expiating is more precautious to clear liability. For more benefit, please refer to Fatwa 88305 which contains a clarification of the amount of Diyah for accidental killing.
To verify if she was or was not a Muslim, he should ask those who are trusted of those who knew her well. It is not sufficient to declare that she was a non-Muslim just because her mother is a Christian who goes to church.
Allaah Knows best.