I think you would find that most taillights are not totally sealed. When you take your bike out of the garage into the sunshine the air expands in the light chamber and pushes out , When you go back to the cool garage it breathes in as the air contracts bringing the moisture in.
Sealing the unit might be futile and frustrating, and one of the fixes is actually to increase the air circulation! If you look around the garage, you might find cars with the same problem.
A similar problem is wheel bearings on boat trailers. The bearing hubs gets hot on the way to the ramp and cool when they go into the water. the contracting air sucks water past any worn seals which leads to premature bearing failure. The fix is to use bearing buddies and to give them a shot of grease just before you launch.