Can I use an AirTag to track my car when its stolen?

Sure. Nothing will stop you from putting an AirTag in a car.

Do note, for it to work, there need to be other Apple devices nearby that can pick it up and relay its location.

So if its taken to an underground parking lot or the middle of nowhere there may not be any device around to pick up the air tag.

Apple is not responsible for the inability to track the AirTag or the car it may be attached to.

For cars there are professional tracking services such as LoJack.