Ok, he's my
to confused matters worse.....
I'm not that familiar with E-body wiring harnesses, I just have some electrical hunches....
My opinion is that the resistance is negligible. You test shows no continuity to each other which is very helpful.
First thing that comes to mind is that a big spark like that is caused buy a large current draw, which means something is GROUNDED. Possibly a unplugged connecter touching metal or though pulling ang pushing of wires possibly the harness insulation has been cut by the frame. I used to see a lot of this when I worked at Acura. It might only be a tiny cut in the insulation leading to a grounded circuit.
You can either try to (IF IT IS A GROUND situation.) put one probe of an ammeter to the positive on the battery then the other to the positive battery cable and see the amperage draw. With a big spark you should see some sort of draw. Someone else maybe can mention here what is acceptable for a static current draw.
With the harness plugged in and the big spark present at the positive terminal you can then disconnect/move wires around in side the car where the problem is localized to see if the amperage draw decreases. If you move things around and the amperage decreases it's sort of "blinds man bluff" because you don't know exactly where the ground may be and if it disappears it may not show up again until probably you're driving a long distance with the wife and kids in the car and you start smelling smoke 200 miles from home.
If you see the amperage draw decrease, focus on that branch of the harness.
Another way to check for ground is that you could use a multimeter (with the wiring plugged in a spark present at the positive terminal) with a probe on a good ground and the other probe on individual harness terminals at the potential area of focus and check for restance to ground. If you get an excessively low ohms reading, that may be the branch of the harness to focus on.
Again, this is only if the harness has a grounded circuit.
Hope this helps!