it is for a convertible, so not everything applies, but it is a decent write up
How To: 5 Speed Swap in Convertibles (and other S13s) : 240SX Convertible Forum

it is the plug on the side of the relay box with two wires. should be a black and a brownish wire. It should be the only plug with two wires going to it. And the wires are a bit more beefy than the others.