Outside mirror sensor
 

Where is the light sensor for the outside driver's side mirror?

Should there even be one?
I would check the back cover of a windshield mounted rear view mirror for such a sensor.

There is a sensor on the rearview mirror but I can't find any confirmation it is also the sensor for the outside driver's mirror. My driver's side mirror is very dark all the time. There must be a sensor since it was not like that a few days ago.

The system is way simpler than you see it,only two sensors involved : 1 rear facing and 1 front facing.
Your mirror may just be malfunctioning,even if it a recent thing.

The signal is sent from your interior rear view mirror

Ditto...the sensors on the interior rear view mirror sends the signal to the side mirrors to dim.

You may want to check the EC (electro-chromatic) connector on the rear of the affected side mirror. The connector is gray/white...and has a purple wire (& black, too I think) running to it.

I knew there is a sensor on the rearview mirror though my search did not make it clear it also controlled the outside mirror.

Thanks.

Just a thought...but I bet you can trick all the mirrors into darkening if you have a bright light (flash light) and something to cover the ambient light sensor (tape?).

The ambient light sensor is on the rear of the interior rear view mirror and faces the windshield. It's the reason the mirrors don't darken during daylight hours and only dims slightly when driving in bright city lights at night.

If you cover the ambient light sensor (during the day) then shine a bright light into the light sensor on the front of the interior rear view mirror...all the EC mirrors should dim as long as you're not in reverse. :)

The reason I was wondering if there is a sensor just for the outside mirror is that the outside mirror is dark and the rearview mirror seems to be normal.

If you look closely at the mirror that is always dark...can you see a bubble...or some type of irregularity?

These mirrors have a conductive gel between 2 layers of glass that remains transparent under normal conditions (daylight). As the sensors detect nightfall (ambient light sensor) and headlight glare (sensor on face of mirror) voltage is applied and darkens the mirror.

Mirror failure can occur when there's a breach and air or water get between the 2 layers of glass...so that's why I ask if you see some irregularity in the mirror glass that could indicate a serious failure.

Also, check the wiring harness to make sure there's not an issue on that end. Have you done any work around the doors that may have affected the wiring that runs from the interior rear view mirror to the side mirror?