The brake lights should be sealed against moisture getting in.
Did rain get inside where the bulb twists in and then installed? Or were they covered and the rain leaked in after installed as it was raining?
The only way I know about getting the moisture out is to open up the brake light by pulling the bulb and let it dry out that way. Faster drying if you bring it inside to dry out, less humidity.
Check to make sure they aren't leaking or when you installed them. Also, did they come from an environment that was cool and air conditioned and then moved out to a hot humid one. Difference in temp and humidity will build up condensation.