How do I fix the mirror on my 206 GTI?

Hi

Mi rear view mirror keeps falling off and even though the surface is clean before re-attaching it doesnt seem so sit flush to the windscreen. I have a 2003 Peugeot 206 GTI with the rain-sensitive wipers so it has the black plastic housing around the base of the mirror. Inside is a metal plate - I've heard that this is the fixing kit and should be on the window rather than inside the mirror? Is this correct? If so, how do I correctly re-attatch it so it doesnt rattle?