I guess that the difference you see is because the car probably uses a 0 thickness single surface for the glass, while windows use glass with thickness.
Sadly, graphic overrides only get you about 90% of the way there: you can override everything to be white but glass surfaces, however it will not work with doors/windows and I would guess objects.
The one time I wanted a 'mixed' white render effect, I ended up setting the windows/doors to white paint and keeping the glass and filtering them out of the override.
So if you don't mind messing with your model a bit to achieve what you are after, it can be done.
Erwin Edel, Project Lead, Leloup Architecten
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