I had this prob on my cruiser a few years ago. I couldn't get into the tail light and I was pretty scared I might ruin the whole
tail light lens. So I just drilled a small hole in the bottom of the light.. away from the wiring and let the water drain out. I drilled it in a spot that if water did get in again it would drain straight back out.. I know it might not be the best solution but that's what I did to my car.