i thinks its purely the heat generated from the roof/spoiler that in a sealed enviroment, the condensation has nowhere to go except collect in the pastic housing. Same as what happens with the xenon headlamps do, steam up after the heat generated and cooling down,causes the lamp housings to sweat, same as the rear spoiler imo.
I dont see the direction of a washer jet nozzle is going to cause the break light housing leak theory.