The Nokia hands-free kit comes in two ways as far as I know, one with a single speaker and one with a switchbox.

The version with the single speaker sends a mute signal to the radio and then uses its own speaker for voice output.

The switchbox version disconnects the speakers from the radio and then feeds its own signal into it.

I wanted the installers to do a mix of that, namely use my already installed speakers (without any additional speaker installations) and to also trigger the mute line of the radio. For some reason that didn't work, they told me switchbox wouldn't have enough power to drive both the speakers and the mute line (which is either a lie or Nokia manufactures crappy equipment).

I don't know if you can feed the voice-out from the hands free kit into aux-in because I think it is already amplified. Maybe you can somehow disconnect the amp though and get the raw signal from the hands free kit. I am nowhere near skilled enough in electronics to find out, however. :)

Till