Am I sinful for riding in and driving a stolen car, knowing it is stolen, and what should I do if harm results from that and I do not know its owner?

You must seek forgiveness for collaborating with the thief and using stolen goods. You are obliged to guarantee compensation for the damage that occurred to the goods while they were in your possession, as well as the rental fee for using them. This is because there are three reasons for guarantee (liability): direct destruction, causing damage indirectly, and the possession of an untrustworthy hand. The owner of the stolen item may claim compensation from either the thief or you. If the thief pays the value of the damaged stolen item to its owners, then the guarantee for what you damaged becomes due to the thief. If you do not know the owner of the right, then give in charity what you owe to them.