Standard MB practice is that a black plastic vacuum line is a vent line. They commonly vent the vacuum system from the cabin, in order to keep the system clean, and not suck up a bunch of junk they just lay behind the dash. Unless your car is unusual, black lines are supposed to leak, they're vents/bleeds. I've been looking for some good diagrams for you, but haven't been able to find anything clear. You should eventually find them just dead-ending somewhere back there.