Thanks for the Factory Service Manual source as this will defiantly help. I am well aware it's going to be a big job but find it curious with the amount of these 2nd gen trucks on the road (Frontier/Pathfinder/Xterra) that there is little to nothing out there in the terms of heater core replacement maintenance posts. Maybe it's such a ***** to do that most resolve this problem by having a mechanic do it."FSM" means "Factory Service Manual." If you do some Googling, you should be able to find a downloadable copy. NicoClub.com has free, downloadable manuals at their site.
The heater core replacement is a pretty big job on these. The entire dashboard needs to be removed to access the heating and cooling unit. I'm pretty sure the air conditioning system will need to be evacuated and later re-charged.