Mercury Mountaineer 2009 Owner's Manual page 489

Entertainment Systems
Battery replacement
Batteries are supplied with the remote control unit. Since all batteries
have a limited shelf life, replace them when the unit fails to control the
DVD player.
Remove the screw and unlatch the battery cover to access the batteries.
The remote control unit uses two AAA batteries which are supplied with
the unit.
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2009 Mountaineer (mnt)
Owners Guide, 1st Printing
USA (fus)