The large temp switch on the thermostat housing grounds the wire at 212 deg F. Take the wire off the switch and ground it; the fan should work.
The fan should also come on when the refrigerant in the A/C gets hot. The switch for this is one the drier at the firewall/pass side. Connect the two wires together and the fan should run.
Bosch Master, ASE Master, L1
33 years MB technician