Ok, do I assume I should leave the battery positive cable on the solenoid terminal A, otherwise there's no power anywhere but the battery, unless we want the resistance of each wire in a dormant state. Is that what we want?
Yes. We want to establish whether or not there are clean circuits here. The multimeter will provide it's own voltage to establish this. We don't need the battery at the moment. We're measuring resistance, not voltage.
If so, the resistance reading between the disconnected battery cable and the cable that goes from the fuse panel to terminal A, when it's disconnected from A, jumps around from 0.1 ohms to 40.## kilo ohms to 0.L.
THIS leg (circuit) looks suspicious. From what you've stated, somewhere "downstream" from here is a problem. THIS is the leg you want to follow.
The resistance for the alternator wire under the same conditions is 13.69 kilo ohms, with the Southwire. It doesn't register with the Actron set at 200 (lowest setting).
This leg appears to be okay.
What I would suggest is leaving the wire lifted at terminal "A" and start testing at the fuse box. We want to prove EVERYTHING good or bad, so I would lift the power wire to the fuse block and test it with the multimeter. Most likely it will test okay but we want to prove
that. Then if it tests okay I would pull ALL of the fuses and signal relays out of the block and test each and EVERY connection where a fuse or relay plugs into. My guess is that one of these connections will have a short to ground on it. (Remember, one wire of the multimeter is attached to the negative battery cable and the other is against what circuit you are testing. Now, IF there is no short to ground, I would attach the power wire back to the fuse block, still leaving it lifted at terminal "A" and testing that point for a short again.
This is going to take time, but I believe you will find the leg with the short.
THERE HAVE BEEN other methods offered by some other knowledgeable posters here and they ARE valid ways to test the circuits. I believe that this is a simpler, but perhaps more time consuming way to locate the trouble. Basically the Idea is to cut the trouble in two and then look each way. Keep eliminating.