To get to the emergency flasher switch, you have to take out the wood panel below the center vent, not the vent itself.

You have to take out the radio unit and unscrew the 2 screws in top of the created opening. If you have a full automatic climate control (with digital display) you need to take that one out as well and you can remove the wood panel by pulling the underside towards you and unhooking the top of the panel (it is inserted/stuck under the center vent.

There are two clamps in top of the center vent and 2 screws at the underside of the vents behind the wood panel (which is the reason why you could not remove the vents).
The wood panel is to be removed before the center vent can be taken out.

greetingz,

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1990 300SL-24
1993 C250D with a minor 600+k kilometer
www.MBenzNL.com(the Netherlands)