These combination fan switches / thermostat controls also suffered from
current overload failure from the current draw of the ac compressor and
the evaporator fan, especially if the electrical connectors were not
secure or if they otherwise didn't make secure connections.  Placing a
load reduction relay in the rear for the compressor clutch helped
protect the switch.

I found the rather large current draw of the ac evaporator fan caused my
fan switch to fail.  Because of the cost, when I replaced the switch I
added 40 amp load reduction relays in the rear on the evaporator housing
and had the original switch to control the relays while the power to the
fan was drawn from the distribution box in the engine compartment.  I
added an inline circuit breaker to protect this circuit.  Making this
change resulted in protection of the fan control switch from overload.
The switch has now lasted 20 years without a problem.

I was amazed the speeds of the fan were much higher because there was
much less voltage drop in the shorter circuit from the distribution box
than in the much longer circuit from the dash switch.  I found the low
speed of the fan was now about the same as the old medium speed; the new
medium was about the same as the old high; and the new high speed was
way stronger than needed for a bus with both the original rear ac unit
and an added dash mounted ac unit.  I rewired the speed reduction
resistors to reduce all the fan speeds so the new high speed was about
the same as the old high speed.

If I had done this before I added the dash ac unit I would probably not
have felt the need for adding in-dash cooling because the high speed fan
would have given enough air circulation for cooling.  Note this is a '86
Westy with the old style ac unit.