My defroster covers interior with moisture

[larryboy_3t5]

Does anyone have an idea why this is happening? I don't smell antifreeze, can't find any leaks, and my windshield doesn't leak. Carpets are bone dry. Can't find any moisture in the cab. But when it rains, and I turn the heater/defrost on it covers the entire interior with moisture. Dash, windows, gauges. It is blowing hot and there is strong air coming from the defrost vents. If I don't turn on the heater at all it is not as bad. I also notice that when I wash the truck, I get small circles of moisture just above the defrost vents. I'm stumped. This truck was stored in a heated garage it's whole life, 17k original miles. It's like a time capsule. The rubber is all very soft, it was like buying a one or two year old truck. everything works like butter. Windows, cold air vents, door latches. This is the only thing that seems out of sorts.