I use to have some of these problems with my 300 SDs so I am assuming its vacuum related.
The vents on the 98 E300 TD are not responding with the CCM controls. I do get them to switch to windshield defrost from the dash vents but not much else. Very little air is diverted to the floor when I change the controls on both the drivers and passenger side to the floor setting. Air is always coming out of the dash vents unless I shut them with the manual thumb wheel.

Any suggestions where I should start trouble shooting first?

Thanks!

__________________
FRED

Daily Driver: 98 E300TD 199K
Hobby Car: 69 Austin Mini
Past Diesels: 84 300SD, 312K
87 300SDL, 251K
94 Chev. K-1500 6.5Ltr.TD, 373K