Air/Fuel Gauge?????'s

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Hey all im new to this but right now i am totally lost in wiring my gauges.... i have just recently put in 2 gauges in my middle air console and they are boost and air/fuel ratio.... i have my boost wired and running and i am having some trouble with the Air Fuel one... i dont know where the O2(oxygen Sensor) Wire is to tap in... if you could please give me some advice on where to go or what to do... i have a red, black, and violet wire that needs to be hooked up and if im correct red- is to the iginition?, Black- Ground?, Violet- O2? please help!
THANK YOU!
Brian