Post Cat O2 sensor, banks 1-3?

[Bill K]

OK, I pulled that error from my service soon light. I understand its the O2 sensor in the back that sits just behind the skid plate. My probblems are these:

There are two sensors side by side and I cant see far enough to tell which is from 1-3 cyl? Is it the one on that faces the drivers side or the passengers side.

2) It looks easy enough to replace, but I cant find the wiring harness connector. I cant see enough where it goes.

Does anyone have a write up with pics for changing the O2 sensors? Anyone done one before? All help is appreciated, thanks

forgot to mention, its a 2001 X5 3.0 liter