HELP. Can't locate Connector

I am putting another engine in my '99 Cavalier. Got the new engine in, trans etc. Had all the connectors labeled, or so i thought. Can't find where this one goes. The blue arrow shows the cable harness it comes from and the other connectors from the same harness that go to auto-trans. I'd appreciate the help. I'm just blind I guess. I thought it came front the underside of the trans but can't see it.

Looks like a plug for the O2 sensor

Where does that plastic attachment push into. I mean the part that would hold it onto the body? I'll look at the O2 . Its a 4 pin connector with the plastic barb end that looks like it pushes and attaches to some holder. In the sheet metal? Thanks

But where physically is the mating connector?

Looks like its the connector that connects to the sensor at the catalytic converter. Will know when I get the drive axles back in and wheels on. Then I'll feel safe to crawl back under it and see. Thanks for the help !