I have a Sony mini-disc head unit. My ipod plugs into the aux in socket via the headphone jack, but that won't work on it's own. It seems my head unit requires a signal to the bus-in to recognise the aux signal. i.e it needs my multidisc changer to be plugged in. This means I can switch to changer mode to hear my ipod.
Alpine, Pioneer and Kenwood all now do head units complete with adapters that allow the ipod to be played through your head unit. They also display the song info and allow full control without having to see the ipod.
Here's a good forum that should help
IpodLounge