Jeff,

You have to remove the lower trim underneath the vents. The trim that houses the controls of the center and side vent opening can be pulled off and underneath you will find some screws that hold the vents.
I don't remember exactly how the top of the vent is fixed, but you either need to get out the compartment box above or you should see some clamps in the top of the vent openings that need to be pushed down in order to get the center vent unit out.

greetingz,

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