GMC TopKick 2007 Owner's Manual page 194

To adjust bass or treble to the middle position,
select BAS or TRE. Then press and hold the
AUDIO button for more than two seconds until you
hear a beep. B and a zero or T and a zero
appears on the display.
To adjust both tone controls and both speaker
controls to the middle position, first end out
of audio mode by pressing another button, causing
the radio to perform that function, or by waiting
five seconds for the display to return to the time of
day. Then press and hold the AUDIO button for
more than two seconds until you hear a beep.
CEN (center) appears on the display.
Adjusting the Speakers (Balance/Fade)
AUDIO: To adjust the balance between the
right and the left speakers, press and release the
AUDIO button until BAL (balance) appears on
the display. Then press and hold the up or
the down arrow to move the sound toward the
right or the left speakers.
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To adjust the fade between the front and the rear
speakers, press and release the AUDIO button
until FAD (fade) appears on the display. Then
press and hold the up or the down arrow to move
the sound toward the front or the rear speakers.
To adjust balance or fade to the middle position,
select BAL or FAD. Then press and hold
AUDIO for more than two seconds until you hear
a beep. L (balance) and a zero or F and a
zero appears on the display.
To adjust both tone controls and both speaker
controls to the middle position, first end out
of audio mode by pressing another button, causing
the radio to perform that function, or by waiting
five seconds for the display to return to the time of
day. Then press and hold the AUDIO button for
more than two seconds until you hear a beep.
CEN appears on the display.
Fade may not be available if the vehicle is a
regular cab model.