hmmm I'm trying to remember what thread it was in. Anyways there's a very good description on here of the vacuum Lines it's worth searching to find but I'll do my best (in short and I hope my memories not faulty) a line comes from the big cannister beside the left headlight and runs back to the drivers side back of the engine. There it connects to a check valve which feeds the interior air direction controls and the cruise control (at least on my 84, the 90 may be different) then connects on mine to CFI manifold.
If it is like mine, if your cruise control works that part of the vacuum is not your air direction problem.