hi,,i did not see your first question posting,,,,may i make an observation ?
In this posting i read your statement,,,
" In theory, you should be able to ground the wire to the thermal switch and that should kick the fan on "
This is an interesting statement,,You say that grounding the wire,,,does this mean that there is 1 wire ?
Because if there is only 1 wire,,,then that,,may not be,,the thermal switch.
The thermal switch,,has 2 wires.
generally a sensor with 1 wire is the coolant temperature sensor,,this sensor sends a signal to the dashboard temperature guage,,or to a temperature light.
The thermal switch is not a sensor and has 2 wires,,1 wire is for voltage into the switch,,and the other wire is for voltage out.
When the coolant is cold,,the contacts in the switch are open,,when the coolant is hot,,the contacts are closed,,this completes the circuit and the fan operates.