this pin, that is available on the belkin car adaptor, was a reasonable candidate to be used to set the radio into the line-in mode when the iPod is connected; unfortunately using the voltmeter i was never able to find a signal on this pin. I expected it to have some internal resistor connected to Vcc and then with the external one form a voltage divider and the resulting signal read by the iPod.
i was stuck on this paradigm and keep searching for a permanent DC signal on pin 21, but after getting frustrated i decided to use the oscilloscope, and after some time and some luck i found the signal.
Here is it without and additional introduction:
It look like the iPod measures the value of the external resistance measuring how long it takes to discharge an internal capacitor. It performs the measurement once every 20 seconds, and two times in a row after the dock connector is just inserted.
No wonder it was difficult to find this signal!!!
Here is a screen shot of what happens after the dock connector is inserted:
This signal is perfect to switch the radio, it happens when the iPod is initially docked, and every 20s thereafter. It is not present when the iPod is off/sleeping. The only drawback is that the return to normal CD/radio operation will happen a little bit over 20s after the iPod is turned off or disconnected.