Replace it with a new one.
Not much detail to go on but the first thing that came to my mind was a leaky heater core. A leaky heater core will fog up the windshield big time.
Actually , the heater core doesn't have a drain
That's probably steam from a leaky heater core.
The 1996 GMC half ton pickup truck heater core can be removed by removing the heater core retaining bolts. Remove the heater core water lines. Reverse the process to install the new heater core.
Sounds like you have a leaky heater core. You will need to replace the heater core and give the cooling system a good flushing, it shouldn't be rusty.
I don't recommend fixing a leaky heater core. Replacing with a new one is your best bet. Heater cores sell for aprox $40 and labor depends on the vehicle.
Take it to a mechanic and say "will you fix the heater core?"
That "mist" is more than likely coolant steam coming from a leaky heater core. Replacing the heater core is your best option.
If that smoke is more like steam, you have a leaky heater core.
The "fumes" are more than likely antifreeze from a leaky heater core.
HEATER WISE? YOUR HEATER CORE MAY BE BLOCKED
You heater works from water flowing from your radiator to the heater core, you may have a leak in your heater core. You most likely also smell antifreeze inside the vehicle. It is most likely casued by a leaky heater core. Unfortunatly depending upon the make/model/year of your vehicle a heater core can be difficult and/or expensive to replace.