GMC Sierra Classic 2007 Owner's Manual page 310

Playing a CD
Insert a CD part way into the slot, label side up. The
player pulls it in and the CD should begin playing. If
you want to insert a CD with the ignition off, first
press the eject button or the INFO knob.
If the ignition or radio is turned off with the CD in
the player, it stays in the player. When the
ignition or radio is turned on, the CD starts playing
where it stopped, if it was the last selected
audio source.
When a CD is inserted, the CD symbol appears
on the display. As each new track starts to
play, the track number appears on the display.
The CD player can play the smaller 3 inch (8 cm)
single CDs with an adapter ring. Full-size CDs
and the smaller CDs are loaded in the same
manner.
If playing a CD-R, the sound quality may be
reduced due to CD-R quality, the method of
recording, the quality of the music that has been
recorded, and the way the CD-R has been
handled. There may be an increase in skipping,
difficulty in finding tracks, and/or difficulty in loading
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and ejecting. If these problems occur, check the
bottom surface of the CD. If the surface of the CD
is damaged, such as cracked, broken, or
scratched, the CD will not play properly. If the
surface of the CD is soiled, see Care of Your CDs
and DVDs on page 347 for more information.
If there is no apparent damage, try a known
good CD.
Do not add any label to a CD, it could get caught
in the CD player. If a CD is recorded on a
personal computer and a description label is
needed, try labeling the top of the recorded CD
with a marking pen.
Notice: If a label is added to a CD, or more
than one CD is inserted into the slot at a time,
or an attempt is made to play scratched or
damaged CDs, the CD player could be
damaged. While using the CD player, use only
CDs in good condition without any label,
load one CD at a time, and keep the CD player
and the loading slot free of foreign materials,
liquids, and debris.
If an error appears on the display, see "CD
Messages" later in this section.