Car Interior synonym?

In your example sentence, just 'car' works fine. The preposition 'in' already makes clear that the subject is not on the roof or the bonnet.

That said, according to Merriam Webster, a cockpit is

4 : a space or compartment in a usually small vehicle (such as a boat, airplane, or automobile) from which it is steered, piloted, or driven

(emphasis mine) so it's applicable here as well.

If you need to specify the position, consider

He sat quietly on the driver's/passenger's/rear seat 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"

The question asked and the example give are at odds and require different answers. The inside of a car is called the “cabin” or the “interior”. (A “cockpit” for a car would only really be appropriate in the context of single-seater or perhaps two-seater, so more a sports car term ex: a roadster.)

In his example, the person could also be in the driver’s seat, so the answer “passenger seat” is not definitive. In the example, words that fit include: car, passenger seat, driver’s seat, and back seat.