The rear part of the ventilation duct has a moulding in the bottom that holds on to the upwards pointing metal sheed of the seat/chassis-beam-opening (dunno the word) it goes through.
The best way to take it out is yto remove the front hallf of the carpet (easier on the passenger's side as on the driver's side), unscrew the Phillips screw that holds the front part of the ventilation duct, remove the front part of the ventilation duct and push back the rear half while pushing the bottom half up.
I don't know for sure but it might be very well possible that there is a Phillips screw that holds the rear half to the front half of the ventilation ducts.

greetingz,

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