If you're car is under warranty take it to the dealer and let them fix it. Your seal is not sealing -- or worse your taillight assembly isn't sealed properly.
If you're out of warranty you'll have to take it out, drain it, dry it, and look for where the moisture/water is coming in and reseal it. Look in the owners manual for the how-to replace light bulbs once you get to that point you'll see the nuts you have to remove to get the assembly out. When you take the assembly out there should be a seal between the assembly and body -- make sure it's intact.
I guess bringing it to a dealer is worth a shot. But when my headlight leaked, thy wouldnt not replace it under warranty. No where would, they said it isnt covered. What you need to do is headup the light so you can pull it apart. Dry the inside with a towel and then let it ait dry for some time, then re-eal it real good and your good to go. If you dont want to go through that work yo can try to take the light out and put a blow drier in the whole till all the water is gone. But thats only a temporary fix.