I want to install sirus radio in my 04 honda accord I want to use the AUX output I don't know how to get to the output and if I need a harness

If you are purchasing your sirius from a reputable dealer, they should have all of the necessary plug ins and adapters you require. Sometimes crutchfield will assist you even though you did not purchase an item from them, and they usually carry all necessary hardware for the install you wish to do, check them out and see if that helps. :)