Mercury Mountaineer 2009 Owner's Manual page 491

Entertainment Systems
To install the batteries, remove the screw at the bottom of the cover.
Then, lightly press down on top and slide the cover off.
When replacing the batteries, use two new batteries (alkaline
recommended) and install them with the correct orientation as indicated
in the battery housing.
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2009 Mountaineer (mnt)
Owners Guide, 1st Printing
USA (fus)