[He does not bear the blood-money of those killed when his car overturned due to a defect in it]

The answer – and Allah is the One who grants success and aid – is that the driver does not owe the blood-money of his passengers. That is because he neither directly killed them nor caused their killing by (culpable) causation, and there is no liability except upon one who causes or one who directly commits (the act).
As for direct action, it is clear that he did not directly kill them. As for causation, it is clear from the question that the cause of the accident was a defect in the car, the danger of which neither the driver nor the passengers were aware.
Moreover, the passengers themselves reassured the driver that there was no danger and told him to continue driving. Thus the driver was not transgressing in his driving, due to their permission and instruction for him to continue.
If it is said: The cause of the accident is shared between the defect that occurred in the car and the driving of the vehicle.
We say: The cause is the defect that occurred in the car; as for the driving, it is only a condition. That is, the defect does not result in the car overturning except on condition that the car is being driven. It is established in its proper place that a condition is not part of the efficient cause (muqtaḍī), i.e., it is not part of the legal cause (ʿillah).

Source: Min Thimār al-ʿIlm wa al-Ḥikmah vol.2