A week ago I was washing my truck. While moving the driver's side mirror housing foward, I heard a "sproiiing" and the mirror pod came loose from its base. I thought I might need to purchase another to replace it. Thankfully, I found Al's (CAPNAL2001) thread on this forum http://www.silveradosierra.com/exterior/side-mirror-problem-t614.html?hilit=side%20mirror#p3099
Al experienced the same issue and later figured out a solution to the issue. Unfortunately, he didn't take pictures during his repair. Fortunately, I did. Al's proceedure was well written, but pictures always help.
Step One: Remove mirror from the pod by tilting it down and to the left, so the lower left corner is at its most inset position. This should allow you to insert your fingers enough to hold the mirror securely. Carefully pull the mirror out. It should release easily.
Step Two: Remove the four Torx headed screws (indicated with red arrows).
Larger image here: gallery/image.php?album_id=1696&image_id=5996
Step Three: Seperate the front and rear pieces of the pod to gain access to the spring retainer.
Larger images here: gallery/image.php?album_id=1696&image_id=5997
and here: gallery/image.php?album_id=1696&image_id=5999
Step Four: Using a hammer, carefully drive the spring retaining ring using a Channel-lock wrench until it seats.
Larger image here: gallery/image.php?album_id=1696&image_id=6000
Step Five: Test the rotation of the side mirror pod. If the retaining ring is set properly, it should not pop off.
Step Six: Reverse the taken down proceedure and re-install the foward and rear portions of the side mirror pod using the Torx headed screws. Do not over tighten the screws.
Step Seven: Re-install the mirror onto the side mirror pod by carefully lining up the white plastic screw bars into their respective gear housings.
Step Eight: Clean mirror of your dirty finger prints and enjoy your repaired side mirror.