Dreaded wet floor pan in Ford F-150?

The passenger side carpet in my Ford F-150 is wet. I know that is a horrible thing because it could be the heater core. But it smells like mildew not coolant, and I have only lost a quart of coolant in the past 2 years. I drive 40,000 miles a year so that's expected. I haven't found any problems online with the 04-14 generation of f150 relating to the the heater core. And I still have hot air, I was reading and saw that the ac condenser can get clogged and cause a problem. Is this the case? Or is it a heater?

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