Set up your green screen behind the car windows you intend to work with. This may involve either placing the green screen inside the car, up against the windows, or beyond the car (from the camera's point of view). Make sure you are not covering anything you want to appear, such as the backdrop behind the car.
Film the scene that the car is involved in, utilizing the green screen in place of the desired interior reflection.
Film or digitally create the "replacement" footage, in this case whatever interior reflection you want shown on the window glass.
Edit your footage, using the keying software to replace the green parts of the primary shot with the replacement footage. This may require careful adjustment of the replacement footage. For example, the car's window frame may cover an important element of the reflection at first placement.
Touch up your merged footage to realistically account for the reflections on the glass that may appear in the initial footage. Ensure the reflections are appropriate to the fullest extent possible, whether that means using the keying software to tweak the replacement footage to account for them, or digitally erasing them if they show something undesirable, such as a reflection of the camera crew.