ok im a little stuck... just got given this AF gauge from my mate when i bought his pilar pods off him..... but.... where the **** does the purple wire go?? (this goes to the o2 sensor bit) Where does it tap into??

Auto Meter Ultra-Lite 2-1/16" Full Sweep Electric Air/Fuel Ratio Gauge Subaru Impreza

any advice please i would be very greatful