In fact, I'm even not sure that internal window glass was present in GTR2 original cars ( I rather guess they came with Race07 ).
That's something which is mostly difficult to create with a perfect result .. much more difficult than the external ones.
I already searched for the original GTR2 cockpit windows ... and never found them separately ( they should appear separately in the .cas file and that's not the case ) or in chassis or cockpit or cockpit_chassis gmt.
Sometimes you seem to find one but looking at it in 3dSimed you quickly notice that the windows are not present.
That's why the original GTR2 windows seem perfect ( except brilliances ) and perfectly transparent .... of course as they are invisible because .... not present.
There are some mods with a very good result ... but rarely perfect for right transparency, brilliances and reflects.
( often rather too milky or too dark or too transparent )
I have not enough knowledge in 3D, just basics, but I guess the base for good cockpit windows is to be found in the "Multipliers" correctly and perfectly used.
I also tried different tricks to get decent ones ( specular, cubemaps, glass color and transparency dds, etc ... ) .... but I never was perfectly happy with them ... they always miss something or have something ... too much.