The first thing to do is to verify where your leak is. When you are wanting the center duct, three things are happening. The defrost is being held closed the diverter flap is applied and the center duct is applied. A leak in any one will send the others to the rest positions.
MB says to remove the heat exchanger to change the center duct; we need to charge more I can see! I don't do assembly work any more and can't say exactly how my guys do it. I think they disassemble the vacuum element and do some laproscopy on it.
BTW to remove the heat exchanger (83-622) the fourth step is to remove the dash (68-100)
Bosch Master, ASE Master, L1
33 years MB technician