What's this vacuum line?

[rhock99@epix.net (Ryan H)]

On my 16V: It runs from behind the firewall, connected to the big black box on the passenger side firewall 'tray' (HVAC controls?).  It goes to the passenger side wheel-well by the power steering reservoir.  It has a T-fitting in-between, and the T'd line goes to a hose right off the back of the intake manifold.
If you need a picture, I have one I can upload.
Thanks.