is it for the temperature sensor at the rear of the block?
That might be the one. Has it got two pins? Does it connect to another loom, or straight into the back of the temp sensor?yes, but iirc the O2 sensor connector is black (at least on my Mk 1.6 Eunos) wheras the temp sensor connector is green, as is the connector on top of the temp sensor. I only know this as I've been poking about getting ready to change the head on my engine.
Does it look like this?...