Transportation Safety Board of Canada - AVIATION REPORTS - 2008 - A08Q0110
In depth discussion of an accident with the EC-120 and how the heat/demist/air/windows should best be utilized to prevent this from happening.
I flew EC-120's in the GOM. Didn't fog up any worse than any other bird.
If you have fog on the inside, cold air (AC is best) will clear it up. If you have in on the outside heat will do it. Of course the outside will clear up quickly after lift off. If you can see to lift off. The inside stuff is more dangerous.