Turning the key to MAR the warning light on the dial turns on, but it should go off as soon as the engine is started.
If the 
warning light turns on when
the car is travelling (on certain versions together with the message on the display),
stop the engine immediately and contact a Fiat Dealership.
See also:
Automatic Dimming Mirror — If Equipped
This mirror automatically adjusts for headlight glare from vehicles behind you.
You can turn the feature on or off by pressing the button at the base of the mirror.Alight
to the left of the button ...
Engine
...
General failure indication
The warning light on the dial comes on when a failure is detected. ...