OK. I am sure you guys don't even have your heater cores hooked up, but, the inside of my windshield is fogging with soap scum when I run the heater or defroster. I replaced the heater core. Didn't fix the problem. Any other ideas? Could it be A/C compressor issues when I run the defroster? Any ideas at this point would be greatly appreciated.

Not sure what the soap scum is referencing but yes, the AC is designed to kick on for defrost (dries the air). the heater does not dry the air, just makes it warm. I am assuming the heat works OK.

If you replaced the heater core, it may have somehow damaged the defrost damper door that directs air across the evaporator coils (AC) and the heat before going to the windshield.

First Check to see if the compressor is engaging when you set to defrost.

bock

Also, Rain-X sells stuff called "Fog-X". With no AC or heater in my car, I run this. It prevents any condensation from sticking to the inside of the car. This includes when I leave it outside overnight.

I have used Rain X and Fog X for racing and off-roading. Amazing stuff!