GMC Sierra 2008 Owner's Manual page 270

DVD/CD AUX (Auxiliary): Press this button to cycle
through DVD, CD, or Auxiliary when listening to the radio.
The DVD/CD text label and a message showing the track
or chapter number displays when a disc is in either slot.
Press this button again and the system automatically
searches for an auxiliary input device, such as a portable
audio player. If a portable audio player is not connected,
"No Aux Input Device" displays. If a disc is in both the
DVD slot and the CD slot the DVD/CD AUX button cycles
between the two sources and not indicate "No Aux Input
Device". If a front auxiliary device is connected, the
DVD/CD AUX button cycles through all available options,
such as: DVD slot, CD slot, Front Auxiliary, and Rear
Auxiliary (if available). See "Using the Auxiliary Input
Jack(s)" later in this section, or "Audio/Video (A/V) Jacks"
under, Rear Seat Entertainment (RSE) System on
page 3-120 for more information.
If a disc is inserted into top DVD slot, the rear seat
operator can turn on the video screen and use the remote
control to navigate the CD (tracks only) through the
remote control.
3-100
Radios with CD and DVD Audio Output
Only one audio source can be heard through the
speakers at one time. An audio source is defined as
DVD slot, CD slot, XM™, FM/AM, Front Auxiliary Jack,
or Rear Auxiliary Jack.
Press the O button to turn the radio on. The radio can
be heard through all of the vehicle speakers.
Front seat passengers can listen to the radio (AM, FM,
or XM) by pressing the BAND button or the DVD/CD AUX
button to select CD slot, DVD slot, front or rear auxiliary
input (if available).
If a playback device is plugged into the radio's front
auxiliary input jack or the rear auxiliary jack, the front seat
passengers are able to listen to playback from this source
through the vehicle speakers. See "Using the Auxiliary
Input Jack(s)" later in this section, or "Audio/Video (A/V)
Jacks" under, Rear Seat Entertainment (RSE) System on
page 3-120 for more information.
In some vehicles, depending on audio options, the
rear speakers can be muted when the RSA power is
turned on. See Rear Seat Audio (RSA) on page 3-129
for more information.