On the Alfa BB, they have this write up for cleaning the mirrors. Those mirrors are the same as Fiatâ€™s IIRC.
â€œFirst, the entire mirror assembly - stalk and all - just pops out of its base above the windshield. Just give it a tug and a twist. Then, the black plastic band around the glass comes off with a little bit of wiggling and prising. Once it is off, you will come to the clear glass protective cover for the mirror. Just take it off and go wash it up. Most of the fogginess will be on it. Beneath the clear glass is the mirror itself. Give the mirror a shot of Windex and a little rub. Take note of the springs inside the mirror, which operate the day/night feature. I recall that they can pop out; but they are easy to put back together.
Now, put the clean clear glass back over the mirror and secure it with the black plastic band. Make sure the plastic band snaps firmly back into place. Go stick the whole assembly back into its base.â€œ
1981 Fiat Spider 2000 (original owner)