The heater core in this model with the Cummins motor and of those around the same year plug constantly. I have to flush mine every couple of weeks in cold weather otherwise it will not keep moister off of the inside windows including the windshield as it just blows frigid air. A complete repair is $2,400+ and it will continue to plug even after the repair. It’s common to need to frequently pull off of the highway and wipe down the inside glass to keep the windows clear.