Mick70 said:Sorry, have an E89 and there is no similar bracket behind the mirror trim :idunno:
Nonetheless, my best guess is indeed M4: At least in my E89 I have found several unused holes at other places with such thread that I could use for funny things. Would also suit good to the screw heads in the picture, in comparison with the voltage module (presume I have the same with another brand).
Best regards
Mick
If Ihad an M4 die I could have cut a new thread on another screw to try. Might have to go and find a hardware shop and try and buy some different sized metric screws to try.