Little PSA here for Flex owners:
Get a small flat bladed screwdriver, the tip should not be wider than 5mm. Stand outside each front door, bend over, crouch down and look up at the underside of the mirror housing and you'll see a rectangular black plastic piece with a notch for the screwdriver. Insert screwdriver and pry off the plastic piece. No damage will be done to the surfaces - nothing to worry about...
If the plastic rectangular patch comes off with no white powder caked on the bottom, put the patch back in place and click it home, you're done.
If, however you see what looks like a secret stash of cocaine, you have the corrosion issue that will eventually require replacement of the mirror housing. Ford or their supplier did not exactly design this mirror to work long-term in the exposure to moisture and salt covered roads. That white cake of crud is the powdered remains of the aluminum or zinc pivot post for the mirror housing. If you are brave, you can remove the painted plastic cover of the mirror and see exactly how badly rusted the mirror pivot is, the coil spring and the metal clip nut that will be sitting loose in the housing - all covered in the white cake dust.
The extra risk is that this corrosion will also spread to the sheet metal of the door where the mirror housing is bolted, bad goes to worse!
In my wife's 2009 SEL, I have had to glue the mirrors down to stop them flopping around. My mirrors no longer fold in - but at least they are solid. I've cleaned out the white corrosion from the mirrors, sprayed them with Rust Check and also sprayed down the sheet metal at the door to prevent rust out of the mirror mounting area.
This is the only area of the car that has rust! I live in SW Ontario and our roads are salted in winter - per normal practice - global warming notwithstanding!