I assume you mean the wire to power the aerial amp? I cant remember the specific colour (I used to do car audio for a living) but once you have eliminated the 8 speaker wires, 3 power wires there should be 1 or 2 wires left. On Nissans (unsure about brand new ones) the rear speakers are on the small plug and the powers, aerial and front speakers are on the larger plug. This applies for pre and post '95 Nissans. (this is when they changed the plug style)
****There is NO earth on the Nissan wiring loom****
On the larger plug there will be 2 wires left over from memory after you find the speakers and powers, one of them is power aerial. You can test by first making sure none have power coming up them, then turning the radio on (you will have fuzzy reception) and touch one of those wires on your stereos aerial wire (blue). If the reception improves thats your wire.
I found a pic of the ISO wiring loom for an older style plug, it looks like the power aerial wire (blue) is directly below the permanent power wire (yellow)!
http://exclusive-part.com/nis-iso2_1.jpgAnd by power aerial I dont mean an aerial that goes up n down, some like the R32 have a booster which just helps with reception but it needs 12v.
I really hope I have answered ya question haha, or I just typed all that for nothing

There are two types of people: Those who divide people into two types, and those who don't