I wonder if it could be the mono valve.
I recently unplugged mine for debugging purposes, and found hot air blew through the defog and side vents even with the climate control totally off. Open may be its 'default' state.
I suppose you could test this theory by crimping off the heater hose temporarily. I have special pliers for this purpose, but vise-grips with suitable padding works too, particularly the needle-nose type.