[Dmitry at Pelican Parts]

Hi Bob - I couldn't find specifically how to tighten the glass, but I was able to find a diagram on their assembly - perhaps this will help you with finding a way to tighten it. Other member will likely chime in with their specific knowledge for this. Hope this helps you a bit.

http://forums.pelicanparts.com/porsche-911-technical-forum/692201-taking-apart-aero-mirrors.html