I also have had both of these symptoms on my 89 260E.
For the defrost vent on with A/C, I found I had a bad vacuum pod. Mine has two separate parts to it and only one half was bad. The result was the defrost vent was left partially open. While waiting for the part, I disconnected the arm from the pod and moved the vent control arm manually to close the defrost vent. Replacing the pod fixed all that of course.
My center vent also does not work. I can see the what I think is the center vent pod up on top of the air mainfold through the glovebox opening, but cannot reach it. I am wondering if it can be accessed by removing the instrument cluster. Another post said it would take 10 hours for a beginner. (So that would be me in that I have never removed a dash before.) Is that really the only way to get to the pod?
I am really interested in anything on how to replace the center vent pod.
John

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John
89 260E 236,000 miles
97 E420 106,000 miles
87 Volvo 765 312,000 miles
07 Volvo S40 T5