from fordexpeditionforum.com (Thermo)
FRK, yes, it is possible to only replace the glass in the mirror. Now, with this being said, it is going to sound like you are destroying the mirror when pulling out the old glass and putting in the new. The inside of the mirror housing, you have 2 posts that are threaded and then you have a center post that is a friction clip. You will need to angle the mirror as far as you can (preferably electrically) to one side or the other (doesn't matter). Then you will want to slide your hand in and pull the glass part a little more and then from the center, pop the center point out of its socket. From there you can pull the first post the rest of the way out and the other post all the way. The mirror is going to make some horrendous noises. It is just the gears slipping past each other.
To install, center the mirror, push the mirror down some so the 2 geared posts get into their holes and start engaging and once you feel the mirror bottom out, push on the center of the mirror some to seat the mirror into the center friction cup.
