I don't have this in the rear lights, but there is a visible waterline on my side indicators. Going to have to try and work out how to get them off, checked and figure out why they are filling up at some point.
I did have this before in another car though, and that turned out to be the seals that had gone hard and failed. I tried to seal them up with all sorts of kit, and never managed to get to the bottom of it, so ended up replacing the lenses.
I've a feeling I may be suffering these same problems soon, as my weather seals need replacing on the B pillars which must be letting moisture inside the cabin. First job to be done for me when it stops bloomin raining!
Cheers
Steve