Using iphone line-out

A couple years ago I made a custom cable to go from the dock connector on my iPod to my car stereo. This utilized the line-out signal from the dock connector, which is the right level for input to the stereo (and higher quality, as it bypasses the headphone amplifier). The cable also charges the ipod when it is plugged in.

I was hoping that I could use this same cable with my iphone, but unfortunately when I plug it in the audio continues to play from the built-in speaker. It does charge, but no sound.

So, does anyone have any information on how to "turn on" the iphone line-out? I would guess I need to drive some pin on the dock connector, which will signal the iphone that an accessory is connected, but I have not been able to find any information about this. Are there any documents for developers designing devices for the iphone that might be able to answer this question?

Thanks,
Alex