Im still googling for the answer, but yes the question is pretty simple for those who do modeling all the time 
at some point of my modeling i have “lost” my windows, they are there but are blank or if im saying this right, they are just simple mesh, also the lighting inside car is messed up but i think i have found answer to that already, will see but if u know what could cause these then jump on your keyboard
I might have it fixed just by clicking halo tab 
If answers is what you’re after, learn to post questions in a way that it’s possible to understand what you’re saying and work the problem. That means using full interface screenshots to support your written questions, and an example .blend of the problem to find the cause.
As is, your simple question doesn’t have an actual question, and the problem description is using quotes.
Like u may see, i wrote that im still googling, this post was to collect something to try to fix it but i will put screenshot now
So, this is inside game, assetto corsa, i can see inside-out but you cant see outside-inside while playing or while in showroom
the blankness has happened during editing this a long time ago 
i just havent had will to fix it but thought i would do it now but my small brain cant fix it i can also put screenshot or the file itself here if you want to have a look at this mess and my horrible 3d skillz