It sound be a Black wire for the ground and a Black-Yellow wire for the power. This BLK-YLW wire hooks up to another BLK-YLW wire that runs to the Fuel Pump Relay and the Air Regulation Soloniod.
Here is the link if you need the wiring diagram.
http://www.240edge.com/manuals/89-90_240sx/l_wiring.pdf
It is on the 9th page and in the upper righthand corner.
