It piggy backs the harness that heads into the rear reverse/fog light assembly in the bumper. It isn't plugged in to anything..?

 

The optional back up warning system

 

Hi

Don't worry too much about spare plugs unless they have a mating socket in the proximity. There are many such plugs for additional option equipment on a D2, it's the way they constructed the wiring looms. I own a TD5 ES7 and there are many plugs around that seem to go nowhere and appear to do nothing. It's the scenario that 'one wiring loom fits all models'.