This happens sometimes. For the 08s and before there is a small slot that you can use a flat head screw driver to (gently) pop off the lens cover. I know that the newer ones have that LED rear fender light, so it might be different. Maybe someone else can chime in.
Just wondering if you wash your bike often? I have been through some torrential rains on mine and have not had any problems.But have seen problems on other peoples bikes that wash them a lot.
The lens doesn't pop off the newer fender tip lights. If you try to pop it off with a screwdriver you will break it. I just had the fender tip light replaced under warranty for the same water-in-the-lens problem, but my dealer would not upgrade me to the brake light fender tip unit.