Chevrolet SUBURBAN 1993 Owner's Manual page 154

Rear Heater (Without Rear Air Conditioning)
K2172
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.
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