The "window" there isn't actually a window, it's just a section of transparent paint. Due to the way DirectX9 works, it doesn't know that it should draw objects behind transparent ones, and as such anything not already drawn behind the "window" is ignored and won't be drawn at all (it thinks that window is solid). The interior of a vehicle is always drawn after the exterior, so you can't see it. The solution to this would be to edit the model files to include a proper rear window, which would be a separate object drawn after everything else, to ignore the problem, or to make the bus yourself.