Nissan was kind enough to basically include both halves of a jumper harness if you have both trucks. I like the jumper method as you leave the engine harness and chassis harness unmodified so future trouble shooting should be easier in theory. So chop off the Frontier engine side connectors and the Titan side of the chassis harness out of the IPDM box then just match them up.
Ok its not that easy. I spent about 3 evenings combing through the FSM's and wiring diagrams I had for both trucks and figuring out what went were is definitely a chore. Ultimately I ended up cutting open the Frontier harness to verify a lot of it since the 2010 FSM and diagrams I had weren't the best at showing connector locations. Also the 2012 Titan FSM was missing entire sections, had to grab 2011 and 2014 manuals and cross check them to sort it out.
Once I make sure everything is correct I'll post up a spread sheet and a text document with notes for anybody else who wants to try it (VERIFY against your year trucks as someone else shared his info with me on an older truck and there are differences). Honestly having done the leg work on this that $350 to send it off doesn't seem so bad
Fun fact about the Nissan FSM and the wiring diagrams I found. They will show a connector, say F14/E5 (plug to each other) and ONLY the wires to those connectors for the exact section you are in. In the engine control section? It only shows those wires. Trans section? Yup, just those wires. When you have a 24-pin connector with like 5 different systems running through it it makes for a lot of going around in circles trying to figure it out
Anyway, I currently have a few wires left to sort out to plug the engine/trans to the truck.
2 wires on the Frontier side are for the t-case and both of them are orange
I already clipped the connector before noticing so I'll have figure that out once I start plugging stuff back in
2 wires are for the t-case actuator. They are flip flopped from Titan to Frontier so need to see which way works correctly. Also one of those oranges goes here.
4 on the Titan side that don't match to anything on the Frontier. 2 are flex fuel, 2 are for what I believe is a heater core bypass valve . The FFV stuff I'll have to add since my Titan was equipped with it. I'll see if the bypass thingie is something that can be added or just cap/delete those wires.
1 ground wire on the Titan side that didn't match to the Frontier side.
1 wire for the oil pressure sensor on the Titan side. The Frontier wire was in the alt harness vs the engine harness so just need a short reroute.
Buy these things if you don't have a set and are stripping out a shit load of wires. Quick and uniform length on the stripped ends. Sometimes if the jacketing is old and stiff you might have to take a couple swipes at it.
I chose to solder my stuff. You can do yours however you feel. I don't care. I've never had a solder joint fail for me personally so I'll keep doing it. That said, the cleanest way would have been to depin the connectors and repin to match but I'm not sure how easy it would be to get the correct pins. 2 of the 3 connectors are different and have different shaped pins entirely, so not just a quick swap either.
These connectors have a ring of solder inside the heat shrink, so just shrink and solder in one easy step
Just make sure you got a good connection with a quick pull after. I found one after that the solder had melted but didn't flow in to both wires. I'll wrap everything nice and pretty once I make sure it all works as intended. I left them a little long in case I totally fucked it all up and have to redo it