Dealt with this problem recently.
For quite awhile the driver's side mirror has had some wiggle to it. A month ago I was headed down the Gorge into a 20-30mph headwind and the mirror shakes were BAD. Stopped and found some painter's tape in the van to hold it in place.
Awhile after getting home set up a time with the dealer to bring it in. No problem, they said something was loose, ended up replacing the entire housing (don't know for sure if that included the mirror part, but I think it did, it was shiny clean).
Great, now I'm ready for a stable view down the side of the van. Nope. Its not quite as bad, but it still has a pronounced shake to it above 45mph or so. The right side mirror is solid and steady. Left side, not so much.
Coming soon I'm off on a four week road trip, not sure I'll have time to get it back to the dealer again. Currently trying some minicell foam tucked along the edge to hold it steady. That might have to do until I get home.