renix O2 sensor testing

Ok, I could crawl under and splice into the feed connector, - or - Does anyone here know the color of the wires into the bulkhead connector on the firewall?
I'm suspecting the large Orange for heater feed, and maybe the grey for ECU input?
I have seen different colors on different PDF files and none specifically referring to the bulkhead connector.
I would love to splice into the circuit up there. It's gonna get hot here soon and I'd rather not be laying in my driveway.
Is O2 wiring on the left (smaller) connector?

Not sure what your trying to do or why? The large orange wire is the heater 12 volt supply. One of the other 2 is a ground, but I had to run multiple meter tests to figure out which of the other 2 wires was ground, and which was the ECU supply voltage, since the ECU uses and internal reference resistor in the circuit.

I'm talking about the inconvenience of checking near the sensor, or ecu.
The wires go thru the bulkhead connector, and that's where I'd like to troubleshoot from...

I cut into mine between the bulk head connector and the O2 sensor connector, near the O2 sensor connector, and added about 24" of wire where it was already exposed near the brake master cylinder, outside of the harness wrapping. I used 12 gauge wire IIRC on the orange wire and 16 gauge IIRC on the other 2 wires, then parked the excess wire in convenient test location. I also used push pin connectors at one end for easy test access..

that's at least encouraging.
didn't wanna go that route
because-
those same wires go thru the large pair of bundles UP TOP..
you didn't happen to document what color they were at the harness connector down under did you?

Mine has 2 small black wires and one large orange wire.

If you are saying your connector NEAR the O2 sensor, that is not what i am asking.
If you are saying I should look for those wires at the firewall on either the left bundle or the right, I will persue it.
My sensor has been disconnected since i bought the Jeep. 30k miles later, when gas prices are rapish, I have reconnected the sensor and would like to know an easier way than testing at or near the sensor, or ECU.
I don't think it's working and quite possibly that is why it was disconnected.

There are two "main" bundles going into what i may foolishly be describing as the "bulkhead" connector. You know, the one with a 1/4 inch screw securing it to it's mate, above the brake master cylinder.

I just want to know which to probe into to determine if there is any proper fluctuation in voltage RE: o2 operation.

I do not want to crawl under (call me lazy) when there is such easy access to these wires from the top.

In fact, I am sure it's not working, smells slighty rich when idling. Oddly enough, my plugs have looked good at every change, and last summer when I could afford to top off my tank during the hottest days, i was getting close to 20 mpg. believe it or not.