Assuming you’ve tried everybody’s favorite Vinegar,
you may need to “scrape” it one good time with a
single edge razor. I had to do this to get the film
off glass shower doors when we bought our house.
Then I went over it good with the vinegar and put a
light coating of car wax. Do not do any of this if
the mirrors are not real glass. Another miracle liquid
is real acetone. CAUTION: Acetone can “craze” many
surfaces, so be especially careful when using it. I’ve
used it to remove painted on lettering from a salad
dressing jar. It’s that strong, so be careful. As a
last resort, you can replace the mirror, but it’s not
a very frugal suggestion.