The rear lamps are most likely to warm up on the hatchback. The peculiarity of their construction is that in damp weather the heat from the lamps is not able to dry all the internal surface of the lamp, because of this, the corners remain dark.
The replacement of the lanterns does not solve the problem, but there is an "craft" solution. There are three exits from the rear of the lamp. Two exits are closed by tubes containing the dust. The third hole located in the lower part of the headlamp shall be released to the passenger compartment. You need to work with the top holes. It is necessary to remove the porous powder and set the normal, denser ones. After that, the problem of the conjecture must disappear.
