I'm getting ready to do a head unit swap myself so I have researched this quite a bit. I have a 2004 Pathfinder Chinook (Canada only model). It is basically an SE with some LE options including the Bose sound system. Unfortunately, replacing the Bose system can get complicated. The head unit has no amplifier. Instead, there's a separate amp for the speakers and as well, the speakers are of 2 ohm impedance rather than 4 ohms as with virtually all other systems. If you just wire in a new head unit, your volume will be way down even if you crank it all the way up. Replacing the speakers is a costly option as the wiring they used for the existing speakers is too thin for 4 ohm speakers. Basically, you have 3 options. Option 1 is to find a replacement head unit and just switch it out with the old one. (Ebay may be your best bet for this.) Option 2 is to order an installation kit including the adapter for the amp and steering wheel controls from someone like Crutchfield. The adapter will allow you to plug the new head unit to the existing amp and speaker system and the installation kit will ease the work of installing the new head unit. Finally, option 3 (May be the most expensive) would be to rewire the entire sound system and replace all the speakers at which point you'll no longer use the existing amp. You would still need an adapter to use the steering wheel controls. My plan is for option 2 as I want Bluetooth and USB capability at lowest cost. Here is a link to Crutchfield - just click on "See the installation gear you'll need." and it will give you two choices - one with the steering wheel control adapter and one without. The site is for Canada, so if you're in the U.S., you'll want to get the U.S. site to get the correct pricing.
https://www.crutchfield.ca/g_300/All-Ca ... html?&pg=1
Hope this helps. Good luck!
Andy