I have a dodge ram pickup and the window fogs ups when I turn the defroster on. Are there any other problem that can cause this other than the heater core

i have a dodge ram pickup and the window fogs ups when i turn the defroster on. are there any other problem that can cause this other than the heater core
Is the vehicle equipped with A/C? If it is, does it work?
The A/C should cycle on periodically during defrost to keep the moisture low inside the vehicle.
If it isn't cycling that, too, could lead to fogging.