The reason they fog up is because the seal is broken around the outside. When you put silicone around them to keep the air out, you are also keeping the air thats inside inside. Therefore, when the air on the inside of the headlights is heated up by the bulbs being on, and the air outside the lights is cold, then there will still be fogging, or moisture, regardless of whether they are sealed or not.
The only way to keep them from fogging is to drill some vent holes on the underside, so that if it rains, or you wash the truck, the headlights dont get moisture in them, however the vent holes are still there, one to let air in, one to let air out. It creates circulation, and keeps the moisture out.
Thats what i did to mine, and havent had any more problems since, and you cant even notice the drilled holes I put in them.
~CYCLONE_06