If you understand the basic operation of the low fan circuit, the single wire terminal on the R15 pre-resistor is the feed for the low fan [ the resistor is in series w/fan motor and that drops the voltage to accomplish a low fan speed].

So if you take that wire off R15 and use it to trigger a seperate relay [ coil side], you can then use a seperate 12V battery V to run the fan on high speed , thereby by-passing the R15.
So, on he contact side of relay [ 30 and 87] , you grab 12v battey source and bring the other side of relay contacts over to the 2 wire terminal of R15..that takes R15 out of the circuit, giing you a high a/c fan vs the stock low fan. It is still triggersed by the a/c pressure swtch circuit b/c you are using that stock circuit and relay to trigger your higher amp relay [ which is required for the higher current load of a high fan speed]

One can also bridge the high side sw S/32 with a cabin sw to give the operator a manual over-ride of the whole fan system, thereby allowing for manual operation when needed [ like towing, traffic jambs, etc]

This over-ride will not change any stock circuits, it simply over-rides those circuits and when not in use , you are back to normal circuitry..a nice feature.

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A Dalton