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I have not taken my steering wheel off in a while, and don't remember exactly how the buttons make contact, but that round metal thing is the explosive cannister for the airbag. Look at all the labels on it. It has those special connectors and what I think are gold plated contacts. It does not make sense that the wires for the steering wheel buttons would pass through the airbag, especially since the airbag is not even made by Mercedes, or that the buttons would use that same type of special connector.
As far as I remember the button connectors are a separate connection that attaches directly to the clockspring. Both those connectors are airbag connectors. I think one is a backup.