hi
well, the handsfree kits made up of a few 'components'
1) microphone
2) cradle / interface to the handset
3) control box
4) wiring to the headunit - the mk5 golf / quadlock headunits are wicked in the sense that they have a telephone input built into them! no need for any fancy wiring, no need for an external 'mute' box, no nothing extra. Where you would connect the 'speaker' in an ordinary handsfree kit, you simply connect it to the L/R connector on the radio connector as it is labelled on the back of the radio - very very easy to do, the handsfree kit comes with the plug to do this usually.
the 'box' i'm referring to is the control box, because the wiring is usually quite standard if its a nokia handsfree kit, so you could in theory just swap the control box for a bluetooth one and that should be that!