How to get rid of condensation on heater core + evaporator core housing freezing

What is your question?

on my 2013 F150 I am having an issue that the technicians cannot figure out. I have great heat when my heat dial is all the way up (to the left) but as soon as I dial it down at all the temperature isn't being regulated and the air is luke warm instead of gradually cooler. The technicians cannot duplicate the issue and assure me the blend door is functioning properly, the only suggestion they have is that possible condensation on the heater core or evaporator core is causing a freeze up. Is there a way to rid this condensation or suggestions for a fix.