If your lens is still 100% then there is only 2 places where water can get into the light.
1) the sealant securing the glass to the plastic may not be sealing properly. I think that this is fairly unlikely as that gluey goo sticks like sheeeeeet to a blanket....
2) inside the engine bay, there should be a soft rubber cover around where the globe fits into the back of the light fitting that has a hole where the wiring plug fits onto the globe. I suspect that this rubber is either not there or is not seated correct.
Either way you need to get the moisture out of the inside of the fitting. There are a few ways to do this. if the fitting has water that you can physically pour out then you will need to remove the light fitting completely and then follow the steps below. If its just mist/fog on the inside of the lens then try this.... Depending on which light fitting it is, if you can get enough space to get your hands in there you can disconnect the plug on the globe and take the rubber cover off the back. Carefully remove the globe without touching the glass of the globe !! Then get the wifes hairdryer out and warm up the glass ( wave the hairdryer around so as not to warm up only one spot of the glass and possibly crack it), while the fitting is nice and warm tie a piece of string to one of those silicone bag thingys that one gets in some packaging boxes that absorbs moisture, and place it inside the globe fitting hole for a few hours. If it falls in don't stress .... it has a string on it....hahaha... Don't shake the light fitting or the bag around with that bag in there as you will scratch the shiny lining in the fitting ruining its efficiency.
Have a pair of tweezers or something ready so you can grip the bag with as you GENTLY pull the string to get the bag out.
Refit everything in reverse order, ensuring the rubber seal is 100% seated.
If you feel it easier to remove the headlight, then a little silicone around the seal between the glass and plastic won't do any harm either.