I'm not sure what type car you have, but I'm replacing the rear O2 sensor in my 2004 Honda Accord and it leads to a connector under the passenger seat. In my case, I had to push the passenger seat all the way forward and pull back the carpet a bit (luckily, it was pre-cut so it wasn't too bad). If you pull out the grommet underneath (where the wire feeds through). you'll see daylight when you pull the carpet back. Good luck!
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I'm not sure what type car you have, but I'm replacing the rear O2 sensor in my 2004 Honda Accord and it leads to a connector under the passenger seat. In my case, I had to push the passenger seat all the way forward and pull back the carpet a bit (luckily, it was pre-cut so it wasn't too bad). If you pull out the grommet underneath (where the wire feeds through). you'll see daylight when you pull the carpet back.
Good luck!
Andy