If you are putting it BACK together, then you have it APART. Seems when I was doing that, I had to do everything 2 or 3 times 'cause I forgot to install 1 thing...
It has been some time since I had my vents out, but IIRC with all the dash stuff out of the front trunk, they connect to the, i'll call it a control box. It has all the cables that direct the air. a small round clip goes around the little tit (red circle). one on the box, one on the vent. might be one on both sides of the opening where the couple.
So the top wedges in under the window and the bottom is attached to the control box. You might be missing the little tit.
Click to view attachmentoh yeah, don't forget the foam seal that goes on the wide funnel. AA and probably 914rubber sell them.