I had this same problem with my Workhorse W22. Went under the hood and found disconnected 2" duct line, and cracked (plastic) vents where the 2" duct line connects. I've had no luck finding replacement vents. I was able to replace the 2" duct line with flexible plastic duct line, restoring dash air conditioning. Inside under the dash the 2" plastic duct line has terrible routing and I would like to redo it all. I have been searching for insulated 2" duct line to replace all of the duct lines I have under the hood and under the dash. This goes for defroster ducts as well.
Picture going down the road in Texas during the summer (105 degrees) with your dash AC on full bore. The AC is making cold air that is then piped via 2" plastic flexible duct under the hood with the outside heat and engine heat, warming the cold air. We get condensation under the dash on the passenger side due to poor routing and thin plastic flexible vents. Think cheapest slinky for the the black tank, but in 2".