Aux input but with a switch?

[X5 Sport]

I have read the articles section and I want to do the same as jmh.which is having DVD changer going through nav on TV and still have a spare aux for my i pod I understand most of it, but can anyone explain a little more about the relay and the switch? Also will the names of the tracks come up on the nav screen if playing Mp3 on the changer or ipod. Sorry if any of these questions seem lame but one of my friends is a bmw tech and will do most of the work for me, but he insists on only pulling the car to bits once otherwise he says I might get rattles. Do I need to get just the standard BMW aux lead? And a list of any other parts I would need would be great.

It is a X5 3.0D Sport 05 also right hand drive just in case that makes a diffrence anywhere

Many thanks.