temp guage reads in the red but the fan stays off!?

my father has an old 96 corsa.
the problem is .... I fitted new temp sensors and thermostatic one but the problems keep coming, the dial in the car reads in the red but the fan never comes on, bridge the wire on the thermostatic switch and it works!
take out the switch at a heat source to the switch and the fan comes on! radiator is hot all round.
help and suggestions will be great.thanks