Yesterday on the way to work in the morning, it was dark and I noticed a foggy area at the top of my mirror. I ignored it assuming it was condensation or something and didn't want to smear it, figuring it would dry off quickly. When I got to work about 50 minutes later, it had grown in size and lessened in effect. After work, it was still there. It is not on the surface, but seemed to be between layers. Also at the bottom, it looked like a slight double vision issue. The auto dim part still functions, when I was stuck in traffic, I would put my finger over the sensor, and it would get bright, then when uncovered with traffic behind me it would green out.

 

That is the nature of the beast.
Few years back I ordered 6 of the auto-dimmers from a yard in NC that said they had 6 in stock.
I gave them $20 each, when I got them, 2 of them were like what you describe, I trashed those 2.

 

It is weird, It has been much colder, it was maybe mid 40s that night, but it has been in the low 20s already. The day before it was fine, just got crappy over night. I have a mirror from a Concord I might try if this one really is shot.

 

Did you tear into them at all?

 

Also, here is a good instructional post on how to ruin a mirror.
DIY: Foggy auto dim rearview mirror.

And here is a decent, but not great video describing how they work.

 

I have had that happen on a calculator screen, which I guess works on the same principle, it's toast and time to get a new one.