Update: All of my vacuum pods are working like they should. An independent shop blessed my work and got my a/c going again. I put the dash back together this past weekend.

So what did I learn? This is going to seem so painfully obvious, but the temperature dial controls the pods just like the buttons do. What's more, the center vent does not blow hot air; it opens only for the a/c. Also, the button with the outline of an arrow pointing up is what gives you a/c (not the one that says "EC").

I also learned that not all control panels are the same. My '87 diesel quite possibly uses a different control panel than its gas-powered brethren, but happily I had one in my parts stash. Unfortunately, the two light bulbs that illuminate the HVAC buttons aren't coming on even after replacing the bulbs. I stuck a test light in the bulb socket, but it didn't illuminate.

Corvo Gold
1987 300TD, converted to manual shift