If you jumper the pigtail leads of the fan switch w/key ON , you should get low aux fan activation..that test the integrity of the low fan/ac circuit..BUT, that test does NOT always mean the sw is at fault. It simply means the relay/fuse/fan. etc test as OK. The most common cause if that test condition is met is a slightly low refrigerant level. That allows for a/c cooling, but not enough high side pressure to trigger the aux fan switch cut-in spec.....that is where the mis-diagnosis comes into play...guys think that the sw is bad when the actual correct diagnosis is low refrig level.
Gauges are the tool ..[ or, in the case of R12, a quick look at the sight glass will tell the tale]

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A Dalton