"Operation of the heater is also affected by the controls on the dashboard. There are four horizontal levers, in three rows.  In the top row, there are a pair of levers that control the volume of outside fresh air allowed into the cockpit. The lever to the left controls two mechanical dampers, one on each side of the cockpit. The lever on the right controls the speed of the fresh air blower, which is located in the center of the fresh air blower case between the control dampers. The center row lever allocates fresh air between the defroster (and the large center vent in the dash) and the under-dash vents. The lower lever allocates heated air between the defroster (and the side dash vents) and the under dash vents."

Quote from PCA NOR'EASTER Online - February 2000 found in its entirety here: Frank Bruns