The ducts and vents are just snap together construction. From your description it sounds like you can just push your duct+internal part towards the dash then snap it onto the external ring.

See the pics for clarification.

On my '83 300CD, there are four plastic "spring" tabs on the outside ring that mate with four slots on the inside of the vent. If you rotate the outside ring while holding the inside vent part stable, it will compress the tabs and you can then remove the outside ring. Don't try to simply pull the pieces apart or you'll break the tabs and/or the slots.

The inside vent part is attached to the duct by four small bump tabs. Pull and twist to separate.

Summary of pics:

Pic 1 shows how my panel looks with the knee panel removed. It may not be factory, but it's my best guess. When I opened this up the first time, it was completely covered by the insulation and padding.

Pic 2 shows the intact slot on the inside portion of the vent

Pic 3 shows the intact spring tab on the outside portion of the vent

Pic 4 shows the broken tab and the broken slot.

Attached Thumbnails