Okay, I took some pictures and gathered a couple more for illustration.

This is a pic of my XJ taken from the drivers side looking down at the intake. The vacuum line on the right side of the pic is the origination of the vacuum connection.

From there it travels along the firewall to where the heater/AC lines go through the firewall there is a "T" connector there that attaches to a line that goes through the firewall in the rubber where the HVAC lines pass through to the interior. If you have a stub of a line there, reconnect it to the "T"

If you don't have a stub, you will need to run a vac line through the firewall, but you won't be able to run it as the factory ran it due to its path.
This first pic (credit to 5-speed) shows the path of the black vacuum hard line from the firewall along the top of the HVAC enclosure. It passes right behind the glove box on the passenger side with a bunch of others.

In the top passenger side of the firewall is a rubber grommet where the wiring harness runs from the interior to exterior. There IS room to feed a hard vac line through there as in mine the PO had ran a bunch of wires through for floodlights and there is still room. You will need to run something through as a "fishtape" to pull the hose through and it will come out in the passenger side floorboard beside the fuse panel. Then route it to the HVAC vacuum connector behind the glove box and tie in to the black line there. That way you have supplied vacuum to the HVAC system.
Pic of the grommet under the hood


This is a pic of the HVAC vacuum bundle behind the glove box I am talking about


I have not tried this, but I see no reason for it NOT to work and it will beat the heck out of having to pull the dash and HVAC bundle to run a vacuum line.

Next is a pic of the vacuum reservoir under the passenger side front bumper. Note: I don't have the original bumper installed and my reservoir is attached differently beside the horns.

 

This post along with another ought to explain the larger hoses. The other one coming from the firewall is indeed the drain for the AC. I have attached a pic of my vent hose and it has the check valve on it, its the big bulbous thing....