This problem is purely a Alpha layer one ... missing or not correctly done.
If this problem is more noticeable through the driver's cockpit view than from outside....
This problem can find its cause in the fact that GTR2 was ( at my own idea ) not yet done for the use of internal windows at the start.... you surely already noticed that the original cars have just an inside banner but no real windows but rather a .... lack of window glass.
When creating mods, we can add internal windows for cockpit view mainly but I already noticed that both window glass have to be different GMTs ... as mostly the needed specular masks and cube maps are mostly different according to the effect of transparency, brilliances and reflections we're looking for.
For mods with same GMT name for inside and outside windshield .... that's more difficult to find a mid-term solution
In your case, I guess you're using a nearly pure white as background color but still not transparent enough.
Using Gimp .... already leaving the opacity at only 5% cannot be low enough for this base layer.
I don't know which painting software you use .... but to create window dds you absolutely need different layers .... glass layer with nearly zero opacity ... and paint layers ( numbers, banner etc... ) with no opacity if you don't wish a see-through or a bit of transparency for a banner for example.
For my own taste, I prefer separating banner from the window GMT and have 2 GMTs in place of 1 so that I can easily can use a purely not transparent banner.
GTR2 is mostly using DXT1 with mipmaps for body, chassis etc ... DXT5 with mipmaps for external windows while Race07 is using DXT5 without mipmaps for internal windows.
( DXT1 doesn't allow transparency ).
As
@jgf explained ... that's a question of own experiments to find the right way to have the most realistic effect.
NB but create a perfect glass effect for outside and inside windows is a real nightmare in GTR2 .... slight imperfections need to be accepted.
