GMC 1994 Yukon Owner's Manual page 147

To operate the rear air conditioning system, the front air conditioning
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 turn the knob to the
blower position you want.
To use the rear control, first turn 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