The rear view mirror has this little black plastic thing clipped to the base, with a wire coming to it from the headliner.

I have the XLT and do not have the auto-dimming mirror or stuff like that,.

So what's the black box for?

 

not 100% sure , but I think the little box you are refering to , mounted in front of the mirror, houses the sensor for the compass.

 

Edit: Nevermind, the compass sensor sounds correct.