The effect of humidity on avionics reliability was researched in the 1960s by the Marconi company. They produced a range of avionics where each box was filled with dry nitrogen and sealed.The advance in component technology (especially capacitors), sealing of printed boards etc. subsequently outweighed any advantage in doing this. I have cautiously put power on old aircraft that have stood outside for up to a year and dripping with condensation inside. A few problems occur, mainly with corrosion or filming on mating connector surfaces. A bit of heat, ventilation and use, usually sorts everthing out.