Shamlessly stolen from someone else....
to all of you with a loose & shaky rear view mirror.
ive solved the problem with mine.
removed roof light. (just prises out).
removed mirror assembly. (2x allen bolts)
onCe removed, you Can see 1x 7mm nut, that holds the mirror to the alluminium braCket.
mine was loose Causing the shaking.
tightened nut, and hey presto, its now as good as new.