Mode Control Lever
WARNING: This page does not describe the selected car, but rather 24 other vehicles, including the 1987 GMC Vandura Special, 1987 GMC Vandura, 1987 GMC Suburban, 1987 GMC Safari, and 1987 GMC S15 Pickup. However, it is still accessible from the selected car via links, so may be relevant.
Mode lever controls a vacuum operated selector valve to operate air door motors, providing desired air distribution. It also allows for selection of air conditioning or heating functions, including defroster air distribution.