I posted this pic a long time ago, and now I can find it using the Search either, so here it is again. Hope it gives you some idea:
How the interior passenger view model was put together. I started with a coach, and built the basic compartment inside it, to get the size and position correct (it has the same frame axis point as the coach). Then I deleted most of the original coach and added lots of detail to the compartment.
The passenger view is a separate model on its own. It can be put into the same folder as the coach, or into a common folder. If you do the latter, you have to edit the .S file and add some path information to the .ace file names. The .wag file has a line that specifies the passenger view model name, and the position and orientation of the camera within it.