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As bad as I hate to say it.. the heater box! I'm not sure you can get to it without it removed. Is the vacuum pod not working?
Years ago.. like 20.. I remember digging some of the firewall insulation out from behind the one on an old driver I had. It was so pinched between the firewall that I couldn't get the hose on there with my fingers..
I think I'm going to avoid removing the heater box for now. I moved the ductwork behind the radio a little I pulled out a black / tan hose. Hooked that to the heater a/c port on the controller. Then I noticed that the vacuum switch in the back had fallen apart. The center pin had broke and the vacuum parts were separated. That selector is only 1 yr. old. I fabricated a new s center pin. It should hold vacuum now. With my luck I probably pulled some wires Louse, and won't find out until I drive her.