Car Interior synonym?

Is there a synonym for a car's interior? E.g. 'cockpit', 'cabin', etc. "He sat quietly in the _____ and waited."

The word that you are looking for is called passenger seat.

Merriam Webster defines it as

the front seat of a vehicle (such as a car) where a passenger sits

Also, we can cross verify it with Oxford Dictionary.

A seat provided in a vehicle for a passenger; specifically (in a motor vehicle) the seat next to the driver

Word Usage:

"His car was released to his fiancée, who was riding in the passenger seat and was sober, Paul said."

"He slouched into the passenger seat of the ivory Ford sedan and shut the door hard"