FORD 2009 F-550 v.1 - Manual (Page 63)

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.
2009 F-250/350/450/550 (f23)
Owners Guide, 1st Printing
USA (fus)
Entertainment Systems
63