If mounting is loose, most likely something more serious than a torx screw that needs to be tightened. My bet is that it is broken from water corrosion. Writer not knowing how it was mounted tells me that he never turned mirror to clean out resevoir that gives access to torx screw and water collected there constantly. If writer now tries to just turn the mirror without exercising great caution may just cause more damage. So turn the mirror (if you can) very carefully. Kostverloren