I took out my Spare wheel/lifted rear deck and reverted to the lower trunk and got a the carpet piece for it as well.  An I found this sensor und the rear seats carpet.

 

Anyone know what it is for?

I don't know what it's for but if you can get hold of the wiring diagram for your vehicle, and you can find your way around one, you should be able to find the answer. 

Damp sensor used in production although Lord knows why, they can also be found on the floorpans beside the cills.

 

Worrying that a vehicle built inside a state of the art factory requires damp sensors, perhaps the days of bodyshells being left in fields to rust are not yet behind us.