Replacing volume knob on '02 325i?

Hi all! I just got an '02 325i, and it is missing the volume knob: it has the white part so I can change the volume, and turn it on and off (it's functional)



So I assume I would just buy a part and stick it on. I haven't been able to find anything to confirm this.. does anyone know how I'd go about putting on a new volume knob? :)