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The other way: I have got a radio with an iPod-input (no other). I want to connect an other audio-device with an cable to it. I connected the 3 audio-pins from the socket with a cable but it don’t work, nothing to hear. The display of the radio only tells “No iPod found”. How can I simulate an iPod to the socket, so the signal “goes through”?

You’ll find multiple interfaces you may be able to use here: http://www.installer.com/iPod/

it sounds like you already know the iPod dock connector pinout - I’ve heard that some accessories look for power provided by the iPod on pin 13 of the dock connector to detect it’s presence. have you checked that out? of course there’s also the serial accessory protocol used by some dock accessories, but I’m not sure if a radio/speaker dock would use that to detect simple presence of the iPod

There is a hardware called DockBoss+, which does this job. http://www.cnet.com/news/dockboss-plug-a