When driving in the dark, the auto-dim mirror was not getting darker to reduce the glare of headlights behind me. So I bought a new GM mirror ($192!), hooked it up, and found it does not dim in the dark, either. There are no bent prongs on either mirror to the wiring connector, and the green light glows. The compass and reading lights work in both mirrors. How can I figure out what's wrong?

Edited: Both mirrors are OEM, exactly the same. Pressing the on/off button does not change anything except the green light.

 

I don't know how your aftermarket mirror works, but the GM has an on/off switch. Far left switch. If that's not it you need a test light to see if voltage is being applied at the mirror.

 

To turn on or off the feature press and hold the button on the left until the green light comes on or goes off as per the manual. If the grn light on the left side is on, it should work.

Here is what the manual says.
Automatic Dimming Mirror Operation
The automatic dimming mirror function is turned
on automatically each time the ignition is started. To
operate the automatic dimming mirror, do the following:
1. Make sure the green indicator light, located to
the left of the on/off button, is on. If it is not,
press and hold the on/off button until the green light
comes on.
2. Turn off the automatic dimming mirror function by
pressing and holding the on/off button until the
green indicator light turns off.
Compass Operation
Press the on/off button to turn the compass on or off

 

Do you have something hanging from the mirror that is blocking the sensor?

 

No. It's a mystery whu the other features work on the mirror, but not the dimming. I am stumped as to why the green light is on, but the mirror does not dim. This station wagon was owned by Avis and rented in Florida. I wonder if the back window was tinted darker so the mirror doesn't get enough glare to dim?

 

When the key is turned on so that it should be working, shine a flashlight on the sensor in the top center of the mirror and see if it dims. I didn't think mine worked properly either until I grabbed my small LED flashlight and gave it the light test.

 

So that's where it went
I have a dark limo tint on my back window, but the dimmer still works fine. I didn't even know I had this until my my first night drive, and was wondering why I wasn't getting blinded.
Owners manuals are sometimes a wealth of general info.

 

I've owned several cars with this type of mirror. You will not notice an immediate dimming of the mirror. The mirror adjusts based on the amount of light detected, but there is nothing to indicate that it has dimmed. It should just seamlessly adjust itself without you really noticing that it is doing it.

 

We had the opposite happen in our Crossfire. The sensor went bad and the mirror was always dark. Didn't bother us any so we just left it that way