pull up on the air vent /slider rectangular assembly bit
You can remove the whole slider thing in one piece to look at it by removing the rectangular plastic bit that it and the air vent are formed into.
Pull firmly from the bottom - on the rectangular plastic surround, (not the slider). At the top of the rectangular surround, there are two plastic fingers that slot into the dash pad so do not try pulling from the top.
The bottom of the rectangular surround is held by those V shaped metal clips LR loves to use - that is what you will be dislodging when you pull from the bottom and tip upwards.
Once off, then you can see what makes the slider thing function, or not. Also if you are looking for a small area to hide things ....