1994 GMC Suburban Owner's Manual - Page 147

To operate the rear air conditioning system, front air conditioning the system must be on. With the front air conditioning system off, the rear system controls can be used to circulate air in the rear of the vehicle. To operate the rear system using the front control, just the knob to the turn blower position you want. To use the rear control, firstturn the front control knob to REAR CNTL. Then, the rear control can be used to increase and decrease the air flow. Rear Heater (Without Rear Air Conditioning) If you have a rear heater (without rear air conditioning),the control switch is located on the instrument panel. To increase and decrease the flow of heated air to the rear floor vents, move the switch with the fan symbol on it to the blower speed you want. Move the switch all the way to the left position to turn the rear blower off, and each position to the right increases the flow of heated air. Move the switch all the way to the right position to turn the blower on high, providing maximum flow of heated air. 3-9