I have a Ford 2005 cassette player (Fiesta MK3 SI) that I would like to keep as it's very much in line with the style of the car. I've also got an iPhone dock on it's way, in dashboard black

The wiring for the charging of the phone is sorted (how to do it: here) but now I just need to get the audio feed from a jack at the back of the dock to the speakers. The radio doesn't have an input, unless I use the tape adaptor, but this project is to remove visible wires and such! What could I do? If it means opening it up and soldering I'm all for it!