Well, I started out tracing a few wires and ended up dismantling almost the entire HVAC system in the Ford E-450 engine compartment. The mechanical stuff was easy enough but the control circuits sure had me talking to myself. I now have a pretty good understanding of the E-van HVAC system. Working on a bus conversion is nothing if not a learning experience.
To answer my own question, the front blower fan and AC compressor are, in fact, controlled by the dash switches - at least on the Aerotechs. In my explorations I found one bad dash switch, a blower fan on it's last legs and a broken fan resistor so it wasn't a total waste of time.
I also rebuilt the van air conditioning and removed 60 feet of AC hose (while cleverly absorbing quite a lot of compressor oil with my T-shirt
). Not bad for someone who didn't know diddly about auto air conditioning three weeks ago.