30-pin Dock Connector and 3.5mm jack
I'm trying the very same thing. I made one myself but the dock station didn't see anything connected so didn't enable the audio signals. I then saw a cable 30pin to 3.5 on the Internet which I ordered and this is doing exactly the same thing. The dock station only uses pin 1 or 2 for signal Gnd and pins 3 and 4 for left and right audio signal.