I have made some tests with "virtual" vehicles. Create a virtual vehicle and attach a dds file as skin works like it should. I tried to attach a skin in a mas-file (skin + windshield) and rFactor creates the vehicle but the ingame skin is just grey (C6R). I have added some talent stuff to the rcd files (made a slow driver ) and that car is 2 seconds off the pace. Perfect! I can add a alt skin to my own car (packed in a mas file + dds files for windshieldin and windshieldout) but it doesn't use my custom windshield .
Sometimes addon skins can be UNdefined in the veh file in car packages... Do you know if you use a car that has a windshield as base it works, while one without it doesn't? Or always doesn't work?