What is this thing? (No picture available)

So, I was looking at my 2005 Explorer today. I recently had remote starter installed. As I was looking at the receiver, now mounted behind the rearview mirror, I noticed that there is a black square shaped thing mounted directly behind the mirror. I have a manual mirror, no compass, lights, etc. However, this thing is plugged into the power cord that would normally power the electric mirrors. My initial thought was that it could be the receiver for the keyless entry. Then I thought it might be a GPS transmitter because my Explorer was a rental car. Does anyone know what this black thing is for sure?

Is the black box part of the rearview mirror housing, or attached to the windshield? Was it there before?

If mounted to the windshield and not there before, then it is very possibly the FM antenna for the remote starter (assuming you have an FM, long range remote start system). Does it have any markings on it?

If it is physically part of the rearview mirror assembly, this could be for the autodimming feature of the mirror if you have that, or possibly part of the auto headlight feature.

-Rod

probably the GPS transmitter