I think the 13 pin round connection/lead is the problem here. I assume the one I have connects to the head unit and exits, as aforesaid, in the boot. As far as I can tell, none of the Sony CD changers on e-bay etc take this 13 pin round connection.
The previous Blaupunkt I had in an Audi also had this connection, but female, not male. You just plugged it in using a similar lead and it worked from the head unit.
Many of the changers also have 2 other wires for audio. I have no facility to plug these in anywhere.