I don't think the termostat has any afect on setings of the fans (main and aux). There probably is a way of modifying this setings. I assume that aux fans have a tempreture sencor somewhere. Also there is a summer termostat that does open at lower tempreture. I just replaced mine due to the same problem that you described. According to the mechanic it was a "summer termostat" but i don't belive him. In any case, two days later I got stuck in trafick and the temp went up again (110C). Now I am seriously considering the switch in cabin to turn on the aux fans, looks to me like the only solution to it. May be Arthur can give us some detail how to do this safely.

Here are the setings for the fans on W124:
Viscousus fan clutch on at 96-104 C
Aux fan on at 107, off at 100.

Dmitry

90' 300CE