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You can easily get high fan instead of low for a/c by using the existing low fan circuit fuses. relay , and wiring, with these changes.
Low fan uses the R15 Resistor , so you disconnect that resistors feed wire [ the terminal with the single wire] and use that feed for a 30 Amp relay coil feed. [ Radio Shack has a nice one for less than $6] . Then ground the other side of coil.
The contacts set of relay will then be fed fused 12V from a good bat. source and the other side goes to the R15 terminal with the 2 wires on it.
That eliminate the R15 low fan resistor and you will now have a high fan speed when a/c calls for aux fan. B/C you are still using the stock low fan relay and sensor circuit, you do not have to do any changes on that part of the sensor circuit. You are simply jumping/eliminating R15.
Some try and just jumper R15, but that circuit has too small of wiring to run a high fans current demand and they will burn up..that is why we use a seperate 30A relay.
The cabin sw is simply put a toggle in the cabin and run a 2 wire zip cord [ lamp cord is fine] and hook each end to the pigtail connectors at the pressure sw . That gives you manual control of fans w/o upsetting the stock circuits..all will still work as factory, but you will then be able to Over-ride the stock fans activation manually...great for towing or traffic jams,etc.
That should be done regardless of a/c modifications, just to have a manual access to fans..very handy when needed..eliminates the old " Turn the heater on " in high thermal conditions when things get creepy....
You can aslo change the coolant sensor by bridging it w/resistor to lower the cut-in spec, but if you have a cabin sw , you can do that before it gets up that high, and if you are using a/c , the high fan will already be On with the relay mod anyway.......