Good point. You may have failed vacuum pods and you're trying to force the recirc on, when the proper fix MAY be to replace the pods and make the system operate as MB intended. It's getting more and more common for the pods to fail as the cars get over 10 years old. It's easy to test the pods with a MityVac at the 7-port manifold behind the glovebox. Now, replacing the bad ones can be a tedious job, but first find out if you need to. Here's a photo of what you're looking for (dash removed for better visibility:
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