Heater Problem

Hi. I hope I am in the right place.

I have a Golf 1.6 FSI Match 57 Plate and have a problem with the heater.

When the heater control is set to the window it just blows very cold air all the time, yet when its set to window and feet it blows hot.

I have had 2 Heater sensors fitted to the left and right hand side and also a black box thing behind the controls by the garage whom I bought it from yet it still blows cold.

Has anyone got any ideas at all what could be wrong.

Thanks In advance.

Paul[]