Mr. Shapiro is right that your question is about civil liability. But because it also relates to criminal law, I will answer it here.

The victim of an attack is allowed to take reasonable steps to defend himself, including the reasonable use of force. He is allowed to cause reasonable amounts of harm to third persons while doing so. But he is liable for any such harm. The attacker is also liable, but that does not absolve the victim.

This may sound unfair at first, but the rule is actually quite just. Let's say that you're the one being attacked, that you damage my car while defending yourself, and that the attacker cannot afford to pay for the damage. Somebody still has to pay for it, and that somebody will be either you or me. You're the one who benefited from defending yourself, so it makes sense that you should also bear the costs of that defense. Besides, you might have had some control over what happened during the fight, but I had none. Making you financially responsible gives you an incentive to do what you reasonably can to minimize the harm you cause to others during your fight. Since a third party like me would have no opportunity to limit the resulting harm, making third parties responsible for it would leave the victim free to cause far more collateral damage than was reasonably necessary.

Of course, if the owner of the car you damaged has comprehensive insurance, he can ask the insurer to pay for it. But the insurer would then have the right to make you reimburse it, and the owner would have a claim against you for his deductible and any other resulting out-of-pocket costs.

Mr. Shapiro is right that your question is about civil liability. But because it also relates to criminal law, I will answer it here.

The victim of an attack is allowed to take reasonable steps to defend himself, including the reasonable use of force. He is allowed to cause reasonable amounts of harm to third persons while doing so. But he is liable for any such harm. The attacker is also liable, but that does not absolve the victim.

This may sound unfair at first, but the rule is actually quite just. Let's say that you're the one being attacked, that you damage my car while defending yourself, and that the attacker cannot afford to pay for the damage. Somebody still has to pay for it, and that somebody will be either you or me. You're the one who benefited from defending yourself, so it makes sense that you should also bear the costs of that defense. Besides, you might have had some control over what happened during the fight, but I had none. Making you financially responsible gives you an incentive to do what you reasonably can to minimize the harm you cause to others during your fight. Since a third party like me would have no opportunity to limit the resulting harm, making third parties responsible for it would leave the victim free to cause far more collateral damage than was reasonably necessary.

Of course, if the owner of the car you damaged has comprehensive insurance, he can ask the insurer to pay for it. But the insurer would then have the right to make you reimburse it, and the owner would have a claim against you for his deductible and any other resulting out-of-pocket costs.