What is this wire for?

I'm working on replacing the old wires in my 83 SR5 4WD 22R pickup. There's a green plug with 5 wires coming out of it: BW goes to the starter motor (switch), YR goes to a sensor right next to the oil filter, (I think it's the oil pressure sensor, it's a big round brass colored sensor with a rubber boot over it), GR goes to the fuel sensor (I think that's what it is, it's a sensor sticking out the side of the engine right behind the fuel pump) YG goes to the water temp sensor the last wire is YB, according to the Haynes manual I _think_ that's the Oil Pressure S/W, but on my truck it is and always has been disconnected. Is this wire really the oil pressure s/w and if so, where on the engine does it plug into? I couldn't find a spot for it anywhere.