autoleads

[J55TTC]

Does anyone know what cable I need, I have a friend whos already done this so it has to be possible.

Hes bought a cable that goes from ISO to a headphone jack but what happens is this:

unplug existing ISO cable (B6 passat standard CD stereo)
plug in new iso cable (has a little box and a lead with a headphone jack to plug into phone)
Existing ISO piggy backs on the new lead so plugs into the back of the new cable
Plug in phone and can listen to my music / tomtom through the stereo.

He explained it to me as the little box makes the stereo think whatever is plugged in is a CD changer.

Any ideas? I called autoleads and they said they only do cables for aftermarket stereos which is bo££ox!