For the roof use reference pics with cars of similar angle.. experiment with highlights unitl you get realistic surface.
And for the interior - imagine that you are building it. So first you should put a windshield as it is deepest thing on the image. Then you put in seat images. Again you can use reference images to position them correctly. Then you should put some basic windows layer. And on top of all you put some highlits and reflections. This way it looks realistic, and you can change seats and add details in interior easy.. good luck