Is it possible that the aux fan will never run if the ac is working properly?
Yes it is possible, BUT only under certain conditions. If you're running your AC in mild weather (let's say 60 f) and the AC isn't being taxed, then it's likely that the low speed fans won't come on. However, if you're running the AC in 100+ degree weather, it is almost certainly going to trigger the low speed fans because it takes more AC compression to keep your cabin cool.
In both instances the AC is working just fine.
'91 300E, 209k miles