if it happens when you run ac/air its because of humidity. maybe air is escaping and fogging up your glass.
If its like that all of the time, could be dirty. oil, or somekind of residue. clean it very well from any kind of product you may have previously used on it.
How long has this been happening? Are you a smoker? It's a puzzle for sure. Could be something in the HVAC system. Have you tried using alcohol to clean it?