It should be 10A fuse #11 in the fuse block inside the cab. You could just get a 12v test light and check for power (key "on") at the white/black wire at the back-up lamp switch's harness connector. If you have power there, then you know the fuse is good. At the same time, you can jump the leads if the connector with the test light and confirm the ground is good, too, which should be if the light was on all the time with the old switch. If that's good, then it's either one of two things: the replacement switch switch is the wrong one (might be the neutral switch instead of the reverse switch) or a bad switch...or, the part inside the trans that depresses the plunger of the switch is so worn that it no longer pushes the switch's plunger in far enough to make it work. If it's the latter, the trans needs to come apart in order to fix the problem. Another option to tearing down the transmission would be to run the wires to a toggle switch in the cab and just the the toggle switch to turn the reverse lights on and off. Really, you would only need to run one wire: you could run the reverse switch's power wire to the toggle switch and just run a short ground wire from the toggle switch to a chassis ground, like the metal support behind the dashboard. I would try getting a new back-up switch from Nissan, first. I've just seen to many problems caused by aftermarket electrical parts.
