I don't know if the center vent on my '86 300E is similar but I'd bet it is. Besides the fasteners you mention, you need to remove the screw adjustment inside the vent. It takes a small hex wrench. You put the wrench in between the vent louvers and unscrew the screw. Then the assembly comes out, but mine was pretty tight.

After replacing the bulb and inserting the vent assembly, you need to replace the adjusting screw. Screw it in until the little air wheel can turn the flap from open to just closed. You'll need a small flashlight to see inside the vent assembly.