Have a look at the vent controls on either side of the heater assembly, you may need to remove trim to get at them.
On the D2 without aircon, the flaps are cable operated from the dash, to linked rods on the side of the heater.
The cable outers are retained by spring clips, you'll probably find that the outer sheath on one of the cables has popped out of the spring clip.
Check that the sheath is in the right position as well, the controls must rotate through their full range of operation while the flaps are operated.
Peter