you're talking about miosture/condinsation on the inside of the head light asenbly correct?
I'm guessing you have a 2000-2002 focus. if you take the bulb out and let it air dry on the hot sun that should cure it for the time being but it will most likely come back. I would remove the head light asembly and use some sylicone sealant on the edges where the clear glass meets with the plastic housing. There is a non air tyte seal somewhere. I know the 2000-2001(or 2002) headlights had this problem.
If you have a new car still under warranty bring it in to the dealership with the condinsation problem happening and you get a new light under warranty.
