It's not the heater fan switch, it's the fan itself. It's the proverbial belly-button of the 240. The theory is they hung a fan on a wire, and then built the car around it. Which considering what's involved with replacing one, seems plausable. There are some interesting cutting tricks that work. Cutting panels to access parts is a speciality of Volvo repairs it seems.
Congrats on the 240 (or did I already say that?). Didn't realize you were thinking of going back that far. To me, the 240 was the last of the rather simple Volvos.