Vectra fan sensor switch?

I have a 1998 2.0ltr 16v vectra, The fan dosn't cut in till almost on the red,
I am trying to locate the sensor switch and have spotted 3 possibilities.
one is in the head near to the temp gauge switch,
the other 2 are in the rad just below the water inlet, one under the other.
I know a bit about cars although not an expert so I would like an accurate answer if poss not just a guess.
Thanks for any help you can give me

Answers:
on the vectra the thermostat opens at 90 and this will allow the cooler water in the radiator circulate and cool the engine the fan should only cut in when your temperature gets to 100 this is just before the red on the gauge, so everything may be functioning correctly. and there are two sensors on the radiator, on the left hand side from the front of the car, should this be the problem then it is the bottom sensor normally. (just so the mechanic above knows for next time)
heard of this before. it was the bottom one on the radiator as far as i can remember. have you tried running it without the thermostat. maybe its seized and cant let hot water past to turn on the thermoswith.
just a thought.
The one on the rad is the culprit

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