1995 993 Tacho - foggy behind the glass

[porsche boy]

I hope you can help with my tacho. On cold days of 10 degrees Celsius or 50 degrees Fahrenheit the inside of the glass is foggy. Tacho only. I have also noted on a normal warm day the glass is VERY slightly grey/foggy on the inside [perhaps stained] Is this a common problem on 993's? What is your suggested fix? I am thinking of removal + clean glass plus perhaps drill a breather hole? Am I on the correct path? Any help would be appreciated. Any removal tips would also be appreciated.