I've downloaded a free 3d car model, and everything seems to be fine except one thing: the windows act very weirdly. When in texture view, some of the faces seem like they are missing, while in the non texture view the surface looks solid. The texture is a solid grey square. I'd be very appreciative if someone could help me to fix this. I provided a few screenshots, so it would be easier to understand the issue.
Non textured and textured views

This is how it's rendering in Blender AND on a game engine (very badly)
