My first post and my first Citroen

It is a 52 plate C3 1.4HDi (92PS) Exclusive with digital climate control.
This seems to work... but there appears to be something missing from the heater housing, which lets air blast into the passenger footwell. I found a picture on Google that shows what I think should be there and a shot of mine for comparison. There is a bundle of wires tucked away behind with a four pin connector that looks a lot like the one on the photo. I assume whatever switch/sensor should be there broke a and a previous keeper never bothered to replace it.
My question - what's missing and what does it do?

Any help appreciated. I tried the local dealer by phone, but it was difficult to explain and they wanted to know what part I wanted - I just don't know!