A slight twist on the already well used and known 'cassette adapter' which slides into a car cassette deck and has a wire + 3.5mm jack hanging out the back to plug into portable CD player/MP3 etc.
Why not have a cassette with Bluetooth (and possibly MP3 player) built in to instantly convert your naff
old tape deck into a state-of-the-art bluetooth compatible mobile 'phone handsfree kit (at a fraction of the price and installation cost of the commercial models) and MP3 player in one?
I'm sure the MP3 bit has already been done, but as yet I haven't seen the bluetooth/phone bit - I had a quick google for this.
For the voice pickup, have a microphone pickup wire where the 3.5mm jack would traditionally be, dangling a short distance out of the tape deck receptacle.
No more annoying earphones/bad sound quality/poor mic pickup etc. etc. - especially for those living in places where handsfree is mandatory for using a mobile 'phone whilst driving (e.g. UK)
Could also be linked with any other bluetooth devices where you'd want the sound played out on the car speakers, i.e. bluetooth iPod adapter dongle etc.