I have a 97 M-Edition that I'm turbocharging, but it's currently N/A. Should I use the O2 sensor closer to the cat (now a resonator) or the one closer to the header? Is there a correct location, or an easier location of the two if they're both okay? Thanks!

 

Once the exhaust flow is merged from all cylinders, the further upstream you are, the better. Admittedly that's coming from an OEM perspective where idle emissions matter too and you want to minimize the time delay between exhaust valve opening and combustion gasses reaching the sensor.

The second o2 sensor on factory cars is mostly an emissions check. All emissions systems need to be monitored for effectiveness so they can self diagnose.

 

About 24" from the exhaust manifold (N/A) or turbo outlet is recommended for a WBO2, lest the high temp kill it.

The controller and sensor that you purchase will give recommendations.

 

For the sake of documentation, I installed it just now at the spot near the header. It can be seen by taking off the left front wheel and looking straight at it. I couldn't get my breaker bar in, so I just put the breaker bar straight out (sticking straight out of the wheel well) with the O2 sensor attachment on, and used a small hammer to smack the attachment. The breaker bar was just to keep the attachment lined up and fully loaded while I was hitting it with the hammer. finally after a bunch of this, it broke free. I snaked the wires up the same way the stock one was, and now I'm about to do some research into how I should route my cable into the car to get under the radio. Thanks for your help! Hopefully this will help someone else in the same situation.

 

I routed my wideband wiring through hole for the shifter, under the main internal rubber boot. Just get some peel and stick foam tape from the hardware store to keep that area sealed up.