Auxy fans not coming on

The auxiliary fans in my new-to-me 1998 SL500 are not coming on. The temp gauge shows a steady little under 100 while driving in hot weather with the AC on, and it will go past 100 after shutting the engine down and turning the ignition on. This is my third SL500, and the fans in the previous cars (1996 and 1998) have always come on around 100 on the gauge.

As I understand it from Alldata, the climate control unit receives engine coolant temp, and AC refrigerant pressure readings. If one exceeds a "known value" it sends a signal to B65, the aux fan relay, to turn the fans on. Alldata says this signal can be tested with "ignition on, both AUTO switches depressed for more than 10 sec" and ">2VDC, fan is running."

As far as I know, there is only one AUTO button on the climate control unit. Anyone have an idea what "both" means?

Thanks,

Chuck