There's (at least) three working items you need to get a signal to the fan.
On the early models, the first is a sensor low down on the rad near the bottom hose (I think that's what you're referring to) - this has two switching temperatures for the two fan speeds. On the later models, the ECU just reads the temperature sensor on the thermostat and has two separate outputs for each fan speed.
These signals operate a relay (one for each fan speed). Depending on year, these may be in the engine compartment or at the top of the fusebox under the steering wheel (one yellow, one black). *If* the sensors above are working OK, you should hear a click from the relevant relay when the correct temp is reached.
The higher speed relay output simply connects the battery to the fan directly. For the lower speed, it connects through a resistor block that's mounted on the top of the fan assembly, on the engine side (rather than radiator grille side).
If the relay for low speed is clicking at the correct temperature but the fan isn't coming on, then this resistor may have blown. Turn off the engine, unplug the wires and use a circuit checker (multimeter or even just a small battery and bulb) to see if a current will flow through it. Alternatively, when you've removed the wires from the resistor connect them together, start the engine and wait for it to wrm up again - you should now find that your fan will come on at *full* speed at temperature at which you'd expect half-speed. It's fine to run the fan like that for a while until you've found a replacement resistor.