While adding a toll responder to inside of my windshield, I took a closer look at the rearview mirror and noticed that there is a plastic arm that extends up to the headrest, but is not attached to the rearview mirror itself, see highlighted area below. Anyone know what function this part serves? Thinking it has something to do with parking sensors or cross traffic alert sensors? But those would be located in bumpers no?

 

This is the rain sensor for wipe.

 

Thanks @hzhao, now I see it in the manual, page 209.