There is a metal “puck� glued to the windshield. The mirror bracket has a spring-loaded clip that grabs onto this puck. The mirror slides downward off of the puck, but depending upon which mirror your have (i.e. manual or automatic w/rain sensor), you may first need to remove the plastic trim. I believe it is the top half of the trim that comes off.
Check this thread:
http://benzworld.org/forums/forums/thread-view.asp?tid=1204247
There's a good photo of the removed mirror there, which may help more than any "instruction" to understand how it is attached.
- RODNEY