Short Answer

Standing is a requirement of prayer. It cannot be skipped without an excuse such as having an injury where you cannot stand. Facing in the direction of Makkah is also a requirement of prayer and cannot be skipped without some legitimate excuse such as being cramped in an airplane. Therefore, sitting in a car for one of the five mandated prayers is not allowed, and you must get out of the car and pray standing while facing towards Makkah. It is allowed pray voluntary prayers in a car without having to stand or face the direction of Makkah since the Prophet did this when traveling.

What the Prophet Said

Ibn Umar said: The Messenger of Allah used to perform an optional prayer on (the back of) his camel in whatever direction it took him. (Sahih Muslim #700a)

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Shaykh Mustafa Umar