Most likely, one or more of the vacuum actuators are leaking under the dash. This is common. There is a vacuum switchover valve located just to the right of the glove box. Remove the glove box and test each line, one by one with a Mighty Vac and you will find the leak. The interim solution for me was to cap off the leaking actuators, in order to stop the heat.

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2000 c280 Sport (looks like an upgrade, feels like a downgrade)
1991 190e 2.3 (sold)