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Hi, I'm Mike G and I'll be glad to assist you. The crack in the stucco may contribute to the moisture in the wall and outlet but the coating on the entire wall is also contributory. Clean the box and paint the interior with a self priming spray paint, caulk the crack with an elastomeric cault, paint the exterior the next time with an elastomeric paint made for stucco, install a new device on fresh stripped wire, cover the box with. A plastic exterior plate. If a receptacle use a GFCI with an inuse cover. Taymac makes one that extends when needed and doesn't look out of place, caulk around the cover. I'm in Florida, renovating a home and totally understand your concerns. The heating of the cover, no doubt was due to the corrosion on the terminations. Seal the box and end the problem