Zantheum

Hey guys. Just had a quick question. I've been looking through some posts and was wondering how i could add an aux-in port/cable for my car? It's a 2007 Aurion Presara with the Satnav 4 CD head unit. I removed the head unit to see if i had any "empty" sockets at the back like im seeing with some of the posts, but all the slots are taken up, im assuming that's because i have the 4 CD head unit. How can i add an aux port/cable without having to buy a totally new head unit?