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.