I had the same issue (cold air blowing through center vent at speed even with blower motor turned off) on my 1993 W201 with 150k on it.
What I did was pull out both center vent grills (grab with pliers protected by cloth on each side of each grill then pull each side loose and out)then remove center panel (4 screws 2 top,1 of these is accessable after pulling rear window defrost switch, and 2 bottom) also a hex head screw just above the lever (inside the slot) in the center of the 2 grills,then work the panel out.
My problem was the connector from the center lever to the flapper door was broken and thus not closing the door when the lever was pulled down and allowing the cold air in.after replacing connector (wrecking yard part) replace in reverse order.
hope this helps anyone else with a similar problem

Gemsbeme
1993 190e 2.6 auto 150k