Answer
In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
In this fatwa:
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.
Answering your question, Sheikh Mustafa Umar, President of California Islamic University, states:
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 Prophet Muhammad (peace be upon him) did this when traveling.
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. (Muslima)
Almighty Allah knows best.
Source: AsktheScholars.com