Well, as per a previous post, I removed the drivers floor mat, got into the vacuum wires, and applied vacuum to all doors, all doors opened and closed, so I believe I've narrowed it down to a leak in the engine bay. There seems to be a check valve in the engine bay near the firewall that connects to the yellow hose, do those ever fail? I'll continue the hunt. Thanks again for all the help.

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2012 550CLS
1982 380SL
1985 300D
1998 ML-320
1997 E-320
1984 380SE