The condensat is in the trunk most often in winter because the temperature in the passenger compartment is much higher than the ambient temperature. Ventilats in the rear racks are condensed to the warm wet air into the trunk where the moisture is condensed on the cold elements of the body.
It is not possible to completely avoid any condensation, but you can reduce it by regularly checking the state of the air vents (in the back wings and the trunk lid) and clearing them from various contaminates. It is also possible to apply a protective layer of anti-cortex to the surface that is most prone to moisture.
