Well my ghetto O2 sensor actually has 2 parts where I have added wire so one is crimped and one is soldered LOL! So I technically still have one non soldered section.No, you cannot solder O2-sensor wire! Use crimp only because the sensor uses the air-gap between wire strands to sample outside air as reference signal.
The -0.1v at ECU shows a ground loop problem. Check all grounds
As for the grounds im not sure where to look. The battery ground? There are 2 wires that go from the battery to chassis. There is this wire that's connected to alternator bracket and some other bracket near the cruise control, but I have no clue what it does. I do have a spoiler light that's grounded to the trunk.
When it comes to wires I probably have no clue where anything goes aside from what's plainly visible..