Yeah, I've already got the headlight assembly out, I am tinkering wtih a couple other things as well. There is not visible moisture. My right hedlight puts out less light than the left. I have switched bulbs side to side and tried adjusting level angles, etc. Someone here previously suggested that maybe fog, or more accurately grim, had built up over years. I don't know, any other ideas?
If you ever get fog/moisture in a headlamp, there's a much easier solution. If you actually have water in it, roll paper towels into thin rods and insert them into the water via the bulb opening. Having the truck leaning so it's all on one side makes this a 5 minute job. Follow with step 2:
If you merely have a moisture fog in the lense, remove the bulb and take an ordinary blow dryer to the back side by propping it in place and leaving it on for a bit. Even a bad case takes no more than 30 minutes. Direct the airflow at the opening, but a bit across it for max results. The entire assembly will heat up and dry.
If you have dirt in there, look for a post from me detailing some experimentation with cleaning the lenses of my Hella foglamps. Can't remember any details, but I spent some time and used various cleaners.