It is a vacuum check valve, but I can't find the part # for the rubber harness. Maybe it's a dealer only item. The valve is under Fast Lane, check part # w0133-1638684

http://catalog.peachparts.com/ShopByVehicle.epc?q=&yearid=1994@@1994&makeid=63@@MERCEDES+BENZ@@X&modelid=6396%3AED|10000072%3AMBC|1530@@E420&keyword=vacuum+valve@@vacuum+valve&catid=All@@All&subcatid=P%3A241861@@Vacuum+Check+Valve&applicationid=W0133-1638684@@Vacuum%20Check%20Valve&mode=PA

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1993 400E, 256,000 miles (totaled)
1994 E420, 200,000+ miles
1995 E420, 201,000 miles