Do you have the repair manual with the wire diagram in it? The one I have shows three terminals used on the switch itself.
Pink (hot) -- to the heater door switch
Yellow/black -- to the heater motor
Yellow -- to the heater motor
The diagram shows:
Pink to terminal 3
Yellow/black to terminal 4
Yellow to terminal 1
You'll want to do a continuity test with a ohmmeter to verify on the switch that those connections makes sense. I don't have a whole lot of confidence I know how to read Alfa's wire diagrams for S3 cars.