Manual mirrors on F-250
 

Hi,

I have a 1999 F-250 Super Duty V10 which has manually adjustable mirrors (i.e. push on the glass to where you want them) Both the drivers and passenger side mirror are loose in their cover, so on the drivers side I now have the plastic cap of a water bottle squeezed between the glass and the casing to keep the glass from moving.
What can I do to tighten up the mirrors? It looks like the casing would separate into two halfs, but I'm not sure, and don't want to take the risk of breaking something.
Thanks!