New Gauge

I'm trying to wire up an A/F Ratio Gauge. I understand the directions except for one thing. It says to connect the purple wire from the gauge to the signal wire from the sensor that goes to the computer. Now I looked up this diagram and there is a black wire commonly refered to as the signal return wire. That one goes from the coolant temp., TPS, Knock, and the EEC test port and there is one on the O2 sensor as well. Now are they talking about that black wire or the dark green wire that goes from the O2 sensor directly into the computer? It's a heated sensor so there are three wires; a dark green, black and yellowish wire. This is an SVO wiring harness if it helps any. Thanks
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The directions say that the gauge should be going from lean to rich during cruising speeds. Mine goes from lean to not even the middle of stoic. during cruising speeds, but it did touch the rich a little when it was under full boost. Thats the only time it went to rich.