Oh dear, looks like quite a challenge, it's turning into more of a job for a professional or an experienced owner, perhaps Cees could take a look if you take him up on his kind offer
All wire colours should match, so you're going to need to trace each wire from the plug to it's destination, using the wiring schematic in the tech manual, i'm assuming either new wires have been spliced in to the old loom, or a new loom has been installed at some time. It's doable, but not easy and could be time consuming, first thing you need is a new meter, as that one is definitely giving incorrect readings, open circuit should read OL, not 1
You'll need to trace continuity by placing one lead on the plug end and the other lead on the destination point, and if it's not where it should be, keep moving the meter lead until you find it, not easy if some are the same or wrong colour
If you're not confident, perhaps time to ask for help, as the consequences of getting it wrong could be disastrous