1995 K1500 Tahoe 6.5L diesel

I have been experiencing issues with my defroster this winter. Turning on the defrost actually causes all the windows and windshield to get fogged up. I dread having to use the defrost setting since I know it will make everything worse before it gets better. After about 10 minutes of driving, it finally clears everything up, but I have to leave it on high the entire time for it to work.

I am noticing a musty smell from the vents as well. There must be excess moisture in the HVAC system.

Here's what I've checked:

A/C Compressor is running
Air is coming out of defrost vents normally
I removed the blower motor to check the evaporator, and it is clean
Evaporator drain is open and flowing normally

What do I need to check next? I suspect an issue with the heater core, but I don't appear to be losing any coolant. Haven't had to add any coolant in months.

Thanks,

Casey