what is the little thing above the rear defrost button, it looks like some sort of sensor?

someone just asked me that today. 99% sure it is the sensor for the climate control.
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It is. It senses the temp, I want to say by means of an infra red system. Someone feel free to correct if I'm wrong.

I'll add to that...

Infrared Dual-Zone Climate control:

It senses both the temperature of the driver and the passenger, as well as the air temp in the cabin. there are also sun sensors!

http://www.jeep.com/en/grand_cherokee/interior.html

It uses infrared technology to sense head/cold distribution in the cabin and adjusts accordingly.