The radiator switch is the first thing to check. It is also important to be sure the system is "burped" so that the switch sensor end is actually in contact with the radiator coolant. You should be able to see if the switch has closed when the engine is hot with an volt/ohm meter or even just a 12V lightblub and some wire.
One caution, I had a local shop replace mine and even though the temperature specs were appropriate (there are two temperature range switches available, mine needed the lower opening variant) the switch they got from their supplier did not open at the specified temperature and was far too slow in opening. If the system otherwise checks out but still doesn't work after you've replaced the radiator switch, it isn't a bad idea to test the replacement in a pan of heated water.
Another tip, If you are overheating in traffic because the low speed fan isn't coming on, turning on the airconditioner should force the high speed fan to come on and the engine should cool down.
However, I thought that you already determined that the fan wouldn't run when you shorted between the switch leads and ground (blk/grn or blk/gry and brn switch connector wires in the diagram above). If the fan won't run when the switch is shorted closed then replacing the switch won't work either. | 
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