Got a question

I have a 97 Ford F350 that has a condensation leak (water) that comes out on the bottom of the evaporator box. It only leaks when I run the heater. There is no anti-freeze, does not appear to be Freon, though I would think Freon would evaporate and not be a liquid.

I have tried to find the drain valve/hose, and can not find it. Not sure if the owner before took it off or if I am missing it completely.

The leak looks like to comes out at the bottom fitting where the A/C line connects to the box. What would I need to do to fix it. Is this the evaporator coil going out? Any help would be much appreciated. Thank you.