Help for hi-fi extra input

I have an 04 freelander sport with an amazing hi-fi system, but I'd like to connect an extra input for mp3 etc.The Harmon Kardon amp. is in the boot so I was thinking of hatching into the wiring harness.I'm not so sure about de-selecting radio or cd to leave the "new" input active...does ayone have any experience of this?...cheers.

Connects 2 do all kinds of adapters to enable ipod connection. Not cheap tho and frequently mean disconnecting any multiplay cd you have.

sub is in the boot...amp is under the passenger seat...