1983 300sd

While upside down under the dash to change the cruise control amp, I noticed an apparently disconnected vacuum line under the driver's side of the dash.

The line in question is a thin black line that comes through the firewall pretty much to the left of the cruise control amp. It comes through a grommet that also contains three much thicker vacuum lines - a red/green, a blue/brown and a brown. I cannot find anything that the thin line should have been connected to (i.e. the other end....)

I've also found a vacuum diagram on this forum that talks about a cabin vent line that is also black, that is connected to the vacuum control valve.

Have I found my cabin vent or have I found a line that should be connected to something else?