Buick 2004 Park Avenue Owner's Manual page 190

Once you have loaded the CDs in the magazine, slide
open the door of the CD changer. Push the magazine
into the changer in the direction of the arrow marked on
top of the magazine.
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Close the door by sliding it all the way to the left. If the
door is left partially open, the changer will not operate
and an error will occur. When the door is closed,
the changer will begin checking for CDs in the
magazine. This will continue for up to two minutes
depending on the number of CDs loaded.
To eject the magazine from the player, slide the CD
changer door all the way open. The magazine will
automatically eject. Remember to keep the door closed
whenever possible to keep dirt and dust from getting
inside the changer.