Ford 1999 Taurus Owner's Manual page 105

short delay. The delay starts when glare goes away.
The delay prevents excessive changing of the
mirrors as traffic moves behind and around the
vehicle, or on rolling hills and curves.
The automatic non-glare mirror
There are two light sensors (photocells) in this
mirror. The sensor that detects rear glare is located
inside a window in the center of the mirror. The
second light sensor which determines outside light
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level is located on the backside of the mirror case.
The driver's side mirror is activated by these sensors
to prevent glare when traffic is behind or around
the vehicle.
NOTE: Do not block the sensor on the backside of
the mirror (i.e., with a parking tag, car deodorizer,
etc.) since this may impair proper mirror
performance.
One important safety feature is that the mirror
automatically returns to the normal position
whenever the vehicle is placed in R (Reverse)
(when the mirror is in the ON position). This
feature ensures a bright clear view in the mirror
when backing up.
Do not clean the housing or glass of any mirror
with harsh abrasives, fuel or other petroleum-based
cleaning products.