Power source for heated O2 sensor

What 12v "switched" power source do I splice into for a 4 wire heated O2 sensor? Key on/key off so I don't burn out the sensor. 91 CRX hf with a b16a swap almost complete.

There's a shit load of 12v switched wires under the hood & in the car. Grab yourself a test light & find the closest wire.

that or you could run it right into the fuse box onto the aux. power sources