So if your P72 ECU is OBD-1, try this...

One of the sensor heater wires should get power from the main relay. That's not really an ECU connection.

There's 3 ECU connections for the O2 sensor.

A6 is the ground side of the sensor heater.
D14 is signal wire from O2 sensor. Maybe that's a shielded wire?
D21 & D22 both should be signal ground, perhaps D22 is signal ground for O2 sensor.