In most cases it's just the ambient moisture that was in the air when the light housing was assembled.
Some quick tricks I've learned:
1. Run your lights on, park in the garage, remove the bulbs (*BE CAREFUL, THEY'RE GONNA BE HOT!!*), and leave them uninstalled until everything cools off, then reinstall them. The warm air inside the light housing will escape and once you put the bulbs back in it should be fine from there.
2. Remove the bulbs, run a hairdryer in the housings to evaporate the moisture. Let it cool, reinstall the bulbs.
3. Wait for a warm day, park the truck facing the sun. Remove the bulbs, walk away for a few hours, when you come back re-install the bulbs.
Basically all 3 methods you're wanting to displace the existing moisture prone air in the housings. I've even seen a guy run a nitrogen feed in the housings and reinstall the bulbs, with everything cold, but my tricks all warm the air up and it generally fixes it. Just be sure your not doing this in an area of moisture ridden air. If you want to be really **** check the barometer first. (Yes, barometer=air pressure but also indicates moisture.)