I started poking around the firewall and found a small cracked rubber hose that splits off from the main and goes through the firewall at about the right place for a vac hose to go to a heat selector box. Upon further inspection, it turned to dust in my hand so... I don't reall know how far it goes through the hole in the firewall to the connection, I don't know if I'm going to have to do an interior tear down to resect a new piece of hose from manifold to control box.
The fun never ends...