FORD 2004 F-150 v.1 - Manual (Page 40)

Do not place objects on top of the instrument panel as these
objects may become projectiles in a collision or sudden stop.
REAR WINDOW DEFROSTER (IF EQUIPPED) R
The rear defroster control is located on the climate control panel and
works to defrost your rear windshield from fog and ice. If equipped, it
also operates the heated mirror to remove snow and ice from the side
mirrors.
Ensure that the ignition is ON position. Press to turn the defroster
ON/OFF. The indicator light will illuminate when ON.
Climate Controls
40