I need to do the same on my 1993 300E2.8, I believe the airbag cover has to come off, there's a bolt under the dashboard in the passenger footwell inside a hole.

Once the airbag cover comes off, I think there's a screw holding the sides of the centre and right side vents in place.

I've not been brave enough to do it, fiddling around the airbag makes me a little nervous.