Nissan doesn't have D+ per see and it is very bad to connect anything to its alternator directly. Euro standard colour coding wiring had you misled as well. That wire is connected to constant B+ via fuse 47. You need wire yelow/black or white/green is for battery light on the dash depends on which model but the that wire is connected to connector marked "L" or pin 3 on the alternator
I do caution you against connecting to that wire directly and which DC/DC charger requires such connection? All the ones I'm aware sense voltage above certain level usually above 13.2V and start charging. There are also voltage sensing relays available on the market which will do the same thing, which are much safer/better solution.
If you still insist then use that connection to trigger a relay which will then supply B+ to your DC/DC charger.