How To Rebuild a Vacuum Heater Control Switch
Excellent post. I'll just add a few of my experiences.
The 5-button switches can still be purchased, though not cheap (~$90). My 65 Newport had the 4-button switch like in the "Touchy business" article. I figured a way to make a 5-button switch work there, by mounting it on an offset metal plate and rearranging some controls. I post all my notes below (ca. 1995). My 69 Dart had the 5-button switch (factory AC). I only get heat in the Newport now with the "rear heat" button, but that is fine.
In all my cars, I added a relay that actuates in blower "high" position. That position most risks melting things, and that happens in many cars. Most 80's M-B owners know of the "melted #8 fuse holder" problem, i.e. cabin blower.