Did the o-ring/spacer/o-ring come on a plastic "plug"? You're supposed to push the entire thing into the fuel line, remove the plastic plug and then press the line onto the rail. If there is no plug, place the spacer on the rail then an o-ring, press the line on, remove the line (o-ring and possibly spacer will stay in the line), install the last o-ring in the rail and push the line back on. Make sure to lubricate the o-rings.
The '90 and up light and harness are different, both power and ground are provided by the harness. I'm sure you can make your light work by only connecting the red wire from the harness.
Well, I just received a Bosch one. Looks identical to the new NTK: metal body, four wires out of the body and reads 4.5 ohms. I will try it and see but I'm not very hopeful.
Edit:
I checked resistance between A&B, 4.5 ohms just like the NTK, ended up just returning it. Interesting that both Bosch and NTK wete made in Spain, they must be from the same manufacture.
In my case when I replaced the sensor with a known "good" one from another MJ everything was back to normal.
Also when I checked resistance of the bad ones between A&B I got 4.4 ohms, it should be 5-7.
I think the new NTK O2 sensors are junk. The old ones had a ceramic body with three wires coming out into the three pin connector, the two new ones I purchased both have metal bodies and four wires coming out of the body into the three pin connector. I have the RenixEngineMonitor from Nickintime and both of the new ones pretty much stay at ~2volts when the truck is warmed up and the REM indicates a lean condition and open loop. I pulled an old one out of my other MJ that fluctuates between 0.2-4.8 volts and lean/rich in closed loop as it should, put it into the MJ that had the new sensor and now everything operates normally. I don't know when they changed, all the pictures I see still show a ceramic body. Anyone else have an issue with them?
EDIT:
This is in my '90 4.0L, "good" O2 sensor was from my '88 4.0L
Factory wiring has full power going through the headlight switch which can overheat and melt/start a fire. The wiring harness Pete mentioned has power running through relays that are activated by the headlight switch instead of power coming through the switch.