Warren
There is a connector in the right sides cover comes from the top in front of the battery. About 1/8 " diameter cord with a RCA ? connector with a rubber cap on the end. The other small wire connecter is for an antenna. Plug your aux feed in and your good to go.
Dusty
yep, is hard to find but is there you will find it eventually amongst all those wires. A cheap AUX jack cord for say mp3 player, etc. say 4 feet long is good to run up under the tank into say what I have is an iron horse tank bib on my gas tank that holds my cheap mp3 player.
Just an fyi, once you figure it out, your music device such as mp3 player has to be CRANKED volume all the way up as well as the radio volume control on your Interstate left handlebar anywhere between 15-22 setting to hear it decent while riding. I love the auto volume control set on HI, works well as speed increases your volume automatically increases and works well, as you slow down the volume goes lower which is nice since less rpms from motor do not need louder volume then at slower speeds.