The only way I can answer you, even though it sounds stupid, is to say that it is a law of physics that you cannot add to the speed of light. That's one of the things that makes light special.
As far as seeing the car goes, you see the car because of light reflecting off of it, so therefore the speed of light allows you to see it. Since light is coming at you from the car (allowing you to see it where it is), no matter how fast it's moving, you can't add its speed to the speed of light.
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