I'm not sure about making it more transparent, but you can make it brighter.
1) Download MAS Editor: rfactor.net/web/dl/rf1/mas_203.zip 2) Open sp_shared.mas (rfactor\GameData\Vehicles\SP Mod\) 3) Extract AE86_Glass.dds (COMMON_GLASS01_DIFFUSE.dds for exterior window tint), Open with a good image editor (photoshop, paint.net <---- Free) 4) Grab the color from the image (it's nearly invisible), hex is: #18191B
5) Slide the V color scale very slightly toward whiter / brighter, delete the entire image and replace with new color. 6) Save, import into mas editor, save again.
This will also work for colors...if you should desire rainbow tinted windows.