Using an additional resistor (resistor block) the fan can rotate at 4 different speeds and smoothly switch between them.
The additional resistor for the heater fan is located on the heater body on the pedal block side.
This is how the additional resistor block is located (view with the instrument panel removed).
Removal
Remove the instrument panel (see here).
Note: It is possible to do this without removing the instrument panel, but in this case you will have to work in extremely cramped conditions, by touch.
Press the clamps, disconnect the connector 1, figure 14-4, of the wiring harness, unscrew the screw 2 securing the resistor 3, remove the resistor from the heater unit (torx T20 replaceable head, ratchet).
Figure 14-4 - Additional Heater Fan Resistor: 1 - Wiring harness connector; 2 - screw for fastening the additional heater fan resistor; 3 - additional heater fan resistor
Installation
Install the removed parts in the reverse order of removal.