There is a transistor based speed controller.
It is housed within the blower assemble (in order to cool its heat sink). The fan is over the passenger foot well & accessible from below.
If it is the speed controller that is at fault, then most people just get another from a scrap yard. If you prefer though & you are familiar with electronics, then there was some discussion of the type of transistor involved a little while ago.
In a right hand drive car, the blower is over the passenger foot well.
There is sometimes, but not always, a piece of carpet trim - held in place by an aluminium tab.
Wires run from the blower motor connections to the controller. The controller is held in by several screws. Undo the screws & pull the controller down & out.
I found this in the archives of this NG. I also had a problem with my heater fan and after dismantling the foot well carpet 3 screws are securing the fan unit and after cleaning the grounding on the transistor, everything was working again and after 2 years it still is Fred (205 GTI 1.9) HTH