It would be a bit odd for sure, but DrinkWater is correct about hitboxes. When a bullet enters a hitbox, it then does an expensive calculation to see if it's going to hit the physics model (in this case the car) and if it hits, then damage can be applied. If we have a hitbox under the chest category (let's say it's the car) and a hitbox under the head category (window area/players) it would be possible to do this. It's just a matter of how much work needs to go into it, and is it really worth it. Also, this isn't going to be accurate at all, like you'd see in grand theft auto games. Don't forget this is the source engine, and vehicles in source are shit.