As fas as an "easy way" to add only an auxiliary input, I'm really not sure. Considering how easy replacing the head unit, and added benifits with it, I would say this is the way to go.
The sound output with the replacement head unit was night and day. SOOO much better than the stock OEM head unit. I was thinking that I wouldn't see a difference, but I was very pleasently surprised.
As for the install and wiring, Crutchfield includes all the wiring harness, and fit-kit panel free with head units (over $125 I think only though). It was a real snap to install, especially for a guy like me who hasn't done it before.
Here are 2 good videos on how to remove the old stereo out of P51 Pathfinders.
VIDEO 1 and
VIDEO 2.
I used VIDEO 1 as a guide, but VIDEO 2 shows how to remove the OEM wiring harness better and illustrates how to disengage the harness triggers.
Hope this helps!