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ditto, but I reversed the hoses a couple times. Did this AFTER using two bottles of prestone engine/radiator flush followed by TWO fresh water flushes. Took about a half hour for the heater core to run clear.

The one that blocks out the light (and outside air, coincidentally). :) I just reread your post. It’s not the HEATER, but the VENT. On the right side of the commander’s seat footwell. Look outside the truck forward of the door and you’ll see a couple of louvers. INSIDE is a control plate...

And make sure the outside vent is closed. Finally occurred to me to do this. Less forced air cooling equals more heat.