you mean there is a spare slot in the quad lock connector that plugs into the back of the RCD300?
i'll do a bit of looking up and get back to you, as i think you may be getting confused with the phone connection that would fill the empty connector
well according to vw diagrams your RCD 300 can infact have 3.5mm aux in and cd changer but they all go through the blue socket , if your stereo was/ is BT compatible it would go into the empty socket next to the blue one and be coloured green
when you say ipod adapter what do you mean just a 3.5mm jack, or the mdi kit? or some other kit/equipment, does the stereo control the ipod?