So I want to ask if there are any tests I can run to figure out what is causing my defrost vents to stay open at full A/C (my 88 300TE).

I have pulled the glovebox and verified the pod is OK. Holds vacuum fine (replaced it a few years back).
I thought about buying a new changeover valve, but then I wondered if it could be the climate control unit, so I'd rather not waste money without trying to test something else first.

Anybody have any ideas on something I can try to figure out what part is not telling the defrost vents to close?

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2016 Monsoon Gray Audi Allroad - 21k
2008 Black Mercedes E350 4Matic Sport - 131k
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2003 Gray Mercedes ML350 - 122k