Humidity while running the heater

2002 Montana. Regardless of the temperature or season, when I run the heat, the cabin is instantly humid and the windows are fogged up within a minute. The floors are dry. The heater core is DRY.
Here's the strange part; with the temperature set high I can stop the humidity by hitting the AC switch on. It will then blow warm DRY air.

there has to be water getting into the car somewhere somehow, turning on the a/c does dry out the air in the interior thats why most cars automatically switch on the a/c compressor in defrost mode to help with defogging the windows

o.k., let's say I have a very slow leak in the heater core (slow as in maybe it's just releasing some steam). Would it make sense then that the a/c control turned on would remove that 'steam'.
The thing I don't like about that theory is that when the heater is on and the air is humid, I don't smell antifreeze and there is no greasy film on the windows.

o.k., let's say I have a very slow leak in the heater core (slow as in maybe it's just releasing some steam). Would it make sense then that the a/c control turned on would remove that 'steam'.
The thing I don't like about that theory is that when the heater is on and the air is humid, I don't smell antifreeze and there is no greasy film on the windows.


Is your antifreeze level decreasing slowly? This still sounds like a heater core.

yeah, a slooooww leak is present. But I can't smell anti-freeze in the 'humid' air. My mechanic's been at the heater core twice and it's been DRY. That's why I'm thinking that maybe it's just a steaming leak once it's hot and under pressure.
The A/C switch still confuses me though. The A/C can't really be running when I have the temp set to HOT and it's cold out....can it?