Thomas: the clear tail light lens are available at the Volvo parts counter at a reasonable price. Volvo knows the problem. I suggest you use clear RTV and also do the other side. The clear lens allow UV degradation of the OEM cement. It does not seem to affect the darker lens.

The front corner lights also have this habit, but are very expensive.

some people have suggested Matt's lenses as a source for used. Do a search. I suggest that RTV be used on all of the corner lights as repair and preventative medicine or "prepair".