The small peice of rubber that sits between the windsheild and the mirror
has fallen off.

Go to your local car yard and look as if your keen on taking a car for a
test drive and remove that one. Problem solved.

Or if you actually have morals (Unlike I seem to with that post!) try
somthing like a small piece of polystyrene or the like and jamm it up behind
the mirror stem. Again popping into someone elses car can show you what your
missing.