As GB states, the aux fan is triggered with the high side pressure switch.
But they have designed the cut-in point of that switch to
be high enough to be tripped only when the a/c is starting to be burdened/taxed with a high thermal load.
It is quite possible to have good a/c operation in mild ambient temps where the high side pressure does not get to this pressure and therefore does not call/need the assistance of the aux fan to bring down the high pressure , [ which is what drawing additional air across the condensor accomplishes].
The true test is comparing the switch/pressure with gauges,
but usually you will see an aux fan cut-in on a hot/humid day in traffic or at idle when there is low airflow on the condensor..
So watch for fan under extreme conditions..
As far as the resistor and circuitry are concerned, you have already verified that as OK with the jumper test. The only thing not confirmed in that circuit , of course, is the sw cut-in spec. and the sw itself....
The same goes for the high speed fan... once again, a safety devise that only gets called for under extreme conditions..
triggered by the engine coolant temp sensor [ 105C on your car]


Thats the basics....