Pretty certain it's a 4mm hex key.

On my cars the center vent only has catches on the left hand side - top and bottom. The right hand side is held in place by a phillips head screw which is accessed by opening the glove box door. Don't know how it works on 124 cars with passenger SRS, the bulb has not burned out on my '93 yet...

I've can't seem to source these lamps at McParts stores. It's dealer or a Mercedes specialist. My local dealer charges ~$4.00 each for those microscopic little lamps! And yes, I understand the economics of dealer parts departments, but it still seems a bit steep.

- JimY