Am I legally responsible for any damage that happens to stolen rentals?

Yes. If you are convicted of grand theft auto or embezzlement of a rental vehicle, as part of your sentence the court can order you to pay restitution to the owner of the vehicle, which can cover the cost of repairing the damage you caused to the vehicle, or the value of the vehicle if you totaled the car. Alternatively, the rental car’s owner may file a civil lawsuit against you to recover the costs of the damages you caused, under the tort of trespass to property or conversion.