Gary,
With a standard GTD radiator it will be in the lower half of the end plate. It is for automatic switch on of the fans however there maybe another thermostat somewhere else doing the job. You really need to check back through the wiring. There is another hole on the passenger side as the GTD is set up for LH or RH drive, perhaps this is being used instead?
I found that this switch (apart from failing twice now) was not very effective. I have fitted a variable setting device to one of the water pipes so that I can adjust the temperature at which the fans kick in.
Unless there is a manual override (which woldbe a pain to use anyway) you need to get to the bottom of this one.