fiesta mk4 - mk4 fiesta 1.25 zetec heater - gasspringmaker

Hi All.

The heater in my fiesta zetec keeps on blowing hot even when the heater control is on cold.!!.

Then it might suddenly cool down and then get red hot again.

Have replaced the heater contol valve aprrox 6 mths ago,

Does anyone know of a way of testing the heater control switch?

Any help will be much appreciated.