From the factory, they are laser sealed (plasti-welded) and cannot be taken apart w/o careful time using a Dremel cutting tool. They are not sealed the same way our headlamps are where a little time and heat can break the seal.
You must have a hairline crack in the body/lens of the light housing somewhere. or perhaps (only guessing) the car was in an accident and aftermarket tails were used during the repair.
You don't say what year and body style your car is, but they are not that hard to remove... a little tight back there is all. IIRC, mine were held in w/3 - 10mm nuts ea. A deep socket and 1/4" ratchet worked well for me. Just don't drop the nuts!!! Then you'll be in for some interior panel removal and a whole lot more time wasted.
Put some double stick tape on the end/edges of the socket.
