a 15-amp single-pole single-throw (SPST) switch across the terminals of the thermal fan switch in the radiator should do the job.
Look at RoberTx's '08 wiring diagram in the link below; you'll see how the thermal fan switch passes the entire cooling fan electrical current when its thermally-sensitive contacts are closed:
CAVEAT: '08 and later only.
Pre-'08? A 10-amp SPST switch between the thermal radiator switch lead and ground works fine.
Now, a question: How do you KNOW your thermal fan switch is defective? Is your fan fuse operational? Fan motor o.k.?
To isolate the problem, JUMP the electrical contacts of your thermal fan switch; fan should operate, whether thermal fan switch is serviceable or not.
Fan still doesn't rotate? Could be blown fan fuse, interrupted wiring circuit, or . . . defective fan motor.
I'd check the fan fuse (ohm-meter) and fan motor (jump + 12 VDC to the hot side) before I replaced the thermal fan switch; that component can be tested by cooking it in a pot of water with a thermometer; contacts should close (ohm-meter) at about 200 degrees F., IIRC.
] Now, a philosopical discussion; your TEMPERATURE GAUGE measures the [coolant temperature at the] cylinder head; NOT the temperature of the coolant [at the thermal fan switch in the bottom of the radiator].
It is possible to have a high cylinder head [coolant] temperature (registered on the temperature gauge) with coolant insufficiently hot enough to activate the thermal fan switch [at the bottom of the radiator].
An AIR POCKET in the radiator and coolant plumbing can cause this condition.
AIR POCKETS can be eliminated by operating the engine 'til the thermostat opens and the coolant circulates in the radiator, radiator cap OFF, while the cooling system is BURPED. After burping, top off coolant in reservoir and radiator, replace radiator cap.