i had this problem. on a warm dry day take your taillights out and let them sit just to dry out the inside once their dry take a bead of just regular home depot clear silicone around the outside edge where the lens means the housing this will close up the very small gap between them and will seal it. i did this to taillights and headlights and they dont get any water or condensation.
Give that a try and if it doesnt work then there is a gap elsewhere probably around the plastic bulb socket. if that is the case i would get some o-ring seals to put on.
+ rep if this helps good luck