for my last 2001 xterra, single disc CD and tape combo head unit, I was able to pick up a USASpec AUX input adapter. If your head unit has a CD changer port on the back (looks almost identical to the normal harness plug) then you can get an AUX in adapter (emulates an external CD changer and pushes aux audio to the head unit)
for those of you with an 02-04 SE Rockford Fosgate stock setup, modding is necessary to get your ipod or aux input. Nissan didn't start adding an actual AUX input in the fosgate units until the 07 xterra, and took until 05 to add a SAT port (aux in adapters available for SAT emulation.)
(see next post) -waltman