Like you did, put another set of O rings around the bulbs, put a layer of silicone on them, or put some Vaseline around the opening on the headlight. That's probably where the moisture gets in. I have to put some silicone on the bulbs in my driver light.
If you do that and it still fogs pull the housing, pull all the bulbs out, run a hair dryer in one of the holes for about 20 minutes, let it cool, then replace bulbs while inside the house (dryer air typically). You may have trapped some condensation inside and it fogs when the inside heats up.
If you do that and it still fogs pull the housing, pull all the bulbs out, run a hair dryer in one of the holes for about 20 minutes, let it cool, then replace bulbs while inside the house (dryer air typically). You may have trapped some condensation inside and it fogs when the inside heats up.