Watch video 1 VIDEO 1

pay particular attention around 4-6 minutes where he demonstrates little resistance but that is with no load and the commentary that follows.

video #2 VIDEO 2

this is a very simplified example to locate voltage drops. There are more videos on you tube

i would start at the battery. Using you're schematic follow the Voltage from the battery checking EACH connection along the way.

the + battery cable carries the V to the Starter motor lug. Sometimes a separate wire off the battery carries the ACC /RUN/lights etc to the starter relay. other cases the wire comes from the starter motor lug .

There is normally a fusible link between the relay and bulkhead connector.

out of the bulkhead to one side of the ammeter

out the other side to your Splice.
out of the splice to the various components some of which are live all the time (lights) others through the ignition switch. MOST but not all will go through the fuse box.
This is difficult at best but Tenacity will win.

I alos wanted to comment on the bulkhead connections. on the male tabs some are visibly making connection evidenced by the female terminal scrape marks. others are not showing those marks that I could see