I know how to get chrome off metal. If your mirror has glass or other parts, I don't know what it will do- I "un-chromed" my master cylinder cover a week ago. It is now the correct gloss black.
Get some muriatic acid (Hydrochloric acid). Home Depot sells it. Folks use it for swimming pools, and it is good concrete cleaner. Mix some 50-50 in a bucket with water and put the chrome piece in for about 10 minutes, and no more chrome plating.
WARNING. The bumper guy who told me how to do it said DO NOT use it on pot metal pieces as it will eat them up. Is your mirror pot matal? Also, this is nasty stuff that puts off noxious fumes when you pour it, so do it outside and don't breathe in much of it. I poured mine out way back in the woods after I was done.