Looks like you got to do the same job I've been procrastinating on . A few years ago I had my dash off and didn't change the hard emissions tubing . Now it's come back to bite me. The vacuum signal supply from the manifold tee has dry rotted from under hood temperatures and have way where it penetrates the firewall . To do it right , I'll have to work from the inside but the glove box is molded plastic on an XJ so the dash must come off ( again). I might try to dream up another short cut but as I remember , the vacuum signal line TO the climate control and line to heater valve sit tightly on the defroster ductwork and may be difficult to pull through . In your case , you can try to remove the center cluster to see if you can manipulate the line if it's not too hard to fish through. The idea is to connect the new line to the old one and pull,it through if there's enough room and the line cooperates. But the problem and symptom your having , loss of floor vents and face vents , only defroster vents is the same as me. Happened to me recently when the temporary repair I made quit due to crumbled plastic tubing. That tube in the photo has a valve to prevent vacuum from bleeding back so when the dash control is selected to off position . That valve should receive its signal from a manifold tee . Look for a connection on the intake that may have an open fitting. Of the hard line going inside the dash is good and only the line under the hood needs repair , you can just find what tubing will fit and run it . I believe it's either 1/8" or 5/16" . But measure to be sure. You want a tight fit but not so tight you can't get it on without fight with it. Should go on with reasonable force . If you have a hand vacuum pump like those one man brake bleeders you can hook it on the the plastic valve and draw vacuum to make sure of the vacuum motors under the dash are working when the lever on the dash control selects different functions. That will confirm if all else works. Hope this helps.
