If all positions switched correctly, there would be only a small amount out of the defroster ducts in most positions other than the defrost position.

Since most of the air vents via the defrost, clearly, the defrost pod is not closing. This can either be the pod, the switchover valve, or the CCU (least likely). The default position is open so you need vacuum to close it.

You can find it by removing the glove box interior. It's on the left side and it's arm points forward and downward...........strange setup. It's easy to replace, however.