Just thought you should know that the low speed will never come on if the a/c is not on . [ which I now see you already knew]
Jim F 's idea of lowering the coolant temp sensor cut-in by parallelling a resistor with the ECT sensors resistance for the high speed fan is something that has worked out well and you may want to do some checking into that.
That is an independent circuit and needs no A/C on to operate.
I have spoken to him about [and am also playing with ]the idea of doing some mods
with the resistor circuit to give high speed to the a/c high side pressure sw circuit.
That seems to be when both the radiator and condensor really
get taxed at high ambient and could use more air flow.
I never did like the idea of a condensor in front of a radiator, but I guess that's where it has to go.....
Anyway, possible resistor change/jumper or cabin switch...
something along those lines.....just thinkin about those real hot/humid areas of the country where the a/c just doesn't cut it.