Replaced Air Vent (+gasket). (Based on some googled descriptions of how.)
Took out several screws in the vicinity of the vent on upper left of door. These are specialized torx screw heads. Released inner panel enough for air vent to slide out. It is held to the panel by the louvered round cover. Twist the cover to the left to remove it and the Air Vent comes loose.
I tested the resistance of the old Air Vent and found it to be ~18 ohms. I had found on one site that it should be between 6 and 18 -- evidently 18 was a bit over proper function.