FORD 2015 Mustang - Manual (Page 79)

To reduce glare, the mirror dims
automatically when it detects bright lights
from behind your vehicle. It automatically
returns to normal reflection when you
select reverse (R) gear to make sure you
have a clear view when backing up.
SUN VISORS
E138666
Rotate the sun visor toward the side
window and extend it rearward for extra
shade.
Illuminated Vanity Mirror
E162197
Lift the cover to switch the lamp on.
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Mustang (), enUSA, First Printing
Windows and Mirrors