How can I install a line-in jack on a 2006 Ford Explorer?

Our 2006 Ford Explorer has a 6-disc CD changer and we wish to supplement that capability with a line-in port, as the FM transmitters that you can pick up at Wal-Mart have proven to be much too cumbersome.

Any insight as on how to remove the radio and hook something up to where the AUX button on the radio takes me to the line-in port?

Note: We just have the 6-disc unit - no rear-seat DVD, no Sirius radio installed.