This has happened to me a couple of times... unfortunately. One thing to check is where the wiring harness semi-screws into the back of the taillight. Put some silicon or some electrical tape around the rubber inserts (where the lights plug into). If you have it completely sealed from the back, then obviously there is a crack someone on the taillight where moisture is entering.
But this took care of one of the two times I've had this problem. Hope it helps!