Ford Mustang Owner's Manual page 59

Under normal conditions, the SELECT control
should be left in any position other than MAX
A/C or OFF when the vehicle is parked. This
allows the vehicle to "breathe" through the
outside air inlet duct.
The Heater Only System (If equipped)
You can heat the inside of your vehicle, defrost
the windshield, or use the vents by adjusting the
three controls in the center of the instrument
panel:
FAN
TEMP
SELECT
The controls for the standard heating only system
To heat the inside of your vehicle, turn the
SELECT control to FLOOR, turn the TEMP
control all the way to the end of the red range
(full right) and turn the FAN control to HI.
After the vehicle is warm, the temperature and
blower speed can be adjusted to maintain a
comfortable temperature.
To defrost the windshield and side windows,
turn the SELECT control to Defrost symbol, turn
the TEMP control to the end of the red range
(full right) and turn the FAN control to HI.
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