The aux fan should always be on unless the AC is in eco mode. Its lowest speed is also controlled by the radiator switch, if the AC controller hasn't already turned it on. Medium speed is controlled by the second contact in the radiator switch (which in my case switches on the main electric fan), and the top speed is controlled by the cluster in 'oh ****' mode
Could be that the aux fan has failed - that's very common on the 4s & 6s. If its not running with the AC on then check to see if its seized. It can also take out the fuse under the passenger carpet if its failing but not actually dead yet.
Add in a worn viscous clutch, and it could get quite toasty!