My son's 1997 1.4 Polo has a problem with overheating.

According to the Haynes book the Polo has a two speed electric fan. There
are three wires going to the fan motor and to the thermostatic switch -
which ties up with it being two speed.

However, the thermo switch has two metal prongs and one plastic one. The
block on the back of the fan also only has two connections. The motor does
run but of course at only one speed given the above.

Has someone put the wrong fan motor and thermostatic switch in the car (for
another model?) or am I missing something?

Many thanks

Mark