I released the two spring clips on the left and unscrewed the allen bolt. the vent came free on the left side but the right side is still attached to something.
on the mbca article of "how to remove", the instructions are only valid for up to '92 model 124. it says to open glove box and remove the screw holding the right side of the center vent. my year does not have a glove box, it has an airbag instead. there is nothing clearly visible on the right side of the vent that I can see that would be holding it in place.
reason I am removing is to replace the light bulb on the thumbwheel.
any ideas?

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.

ok, I'lltake on the job. I'll disconnect the battery & hopefully only need to remove one bolt for the air bag to drop.
will post if job succeeds

Be sure to disconnect the battery first. The passenger airbag is held in place by a single, long, bolt that is screwed in vertically. If you place your head in the passenger footwell and look upward, you'll see a hole in the under dash "kneepad." Several inches up that hole is the airbag retaining bolt. You'll need a 10 mm socket on a six inch extension. Once that long screw is out you'll need to wiggle out the facia panel which is integral with the airbag assembly. On my 1993 W124, the two wires that attach to the back of the airbag/facia assembly were slightly different in color and tracer, so keep track of your wire polarity. It is a bit of trouble just to get to that one little screw that retains the center vent. Incidentally, the vent's fins are a bit frail so don't pull too hard on them. I fished a piece of clothes line into the vent, behind several fins and then back out again and used it as a "handle" to GENTLY pull as I released the clips on the left side of the vent. Good luck.