If you are blessed enough work with special needs people, and one of them poops in the back seat of your new car and the seats are fabric, you can wash it with soap and water at least 8 times. You can even try baking soda and vinegar, then more soap and water the next morning. And still, you can’t get to smell to go away.
Take the seat out and put it in a room in your house which you don’t use very often, and the flies will stay in there.
If you can get the flies to go in your car instead, you can ask everyone: What has four wheels and flies? When they give up, tell them your car has four wheels and flies.