The bad news: It's not the switch as I thought, but a relay on the blower motor. Expected cost, parts and labor inclusive, is going to be about $550.
The not-so-bad news: The part is readily available and they may even have it finished late this afternoon, but if not, certainly by mid-day Monday.
I'm obviously not thrilled about the cost, but I need the heater to work, so there's not much avoiding it.