GMC Sierra Classic 2007 Owner's Manual page 326

Listening to a DVD
If your vehicle has the Rear Seat Entertainment
(RSE) system and a DVD is playing, the DVD
symbol appears on the radio display indicating that
the DVD is available and can be listened to
through your vehicle's speakers.
To listen to the DVD, press the CD AUX button
until RSE appears on the radio display. The
current radio source stops and the DVD sound will
come through the speakers.
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To stop listening to the DVD, press the CD AUX
button, if a CD is loaded, or press the BAND
button to select a different source.
When the RSE system is turned off, the DVD
symbol goes off of the radio display and RSE OFF
appears on the radio display. The radio returns
to the last radio source that you were listening to.
See Rear Seat Entertainment System on
page 329 for more information.