air/fuel meter

I want to install a autometer z series air/fuel gauge in my 2002 cavalier. The gauge has 3 wires; 12 volt ignition, ground, and sensor wire. I know where my sensors are in the car. I am not sure If I should use the upstream (before cat) or downstream (after cat). I know that those sensor wires run to my PCM and I know where that is. I know that the upstream sensor has a purple wire (which is the sensor wire). The only problem is that there are more than just 1 purple wire that goes to my PCM. I cant really trace the wire loom that runs from the sensor to the PCM, because for one, the loom connects with other wire looms along the way, and 2nd, the wire loom goes under parts of the engine and other parts and is basically impossble to get my hand under. My question is first do I use the upstream or downstream sensor? Also, can someone please tell me how abouts doing this without frying anything. Thank you in advance.

I got the air/fuel gauge because I have a 3 gauge A pillar pod that currently has 2 filled (volts and vacuum) and needed another gauge to fill it up. I really dont care if its accurate or not. Yes the sensors are factory. I was thinking upstream was the sensor I needed, but i thought I would ask (better safe than sorry). Ok, so now that I know which sensor to use, I know know what wire I need to look for (purple). My problem is that the PCM has 2 plugs. Both plugs have atleast 1 purple wire each. I tried following the wire loom from the sensor to the PCM and its a total nightmare. The wire loom "T's" off, and also goes under parts of the tranny and other electronics so I really cant follow it. That is why i am trying to find out which plug it would be so I can work on this alittle better.