ok, update on this for anyone else thinking the same.
I got a KM20 FM transmitter thing. The bluetooth on it is garbage as the signla level is too low, but plugging a USB pen into it and then transmitting to the car stereo, it is great. Can't see what song is playing or anything as the lighter socket is too far down on the Jazz, but, the sound is certainly adequate and I can put loads of albums onto the pen. Great stuff.
One drawback.
Went to my car today for the first time in a few days and the battery was totally flat. I noticed that the gadget is still in the lighter socket so I'm assuming it must have drained all my juice away. I'd always thought that if the keys were out of the ignition then nothing would be taking juice from the battery?