Oxygen Sensor
The MR2 Turbo has an O2 sensor located in the exhaust elbow between the turbo exhaust outlet and the primary catalytic converter. The sensor helps the ECU control the air-fuel ratio. The sensor is a four-wire sensor. The 2 black wires are for the sensor heater, the white is the O2 sensor output, and blue is the ground.
Owners who install air-fuel gauge typically tap into the white and blue wires. You may want to confirm this information with other owners before proceeding as my information is not by my experience.