Turning key to "run" causes all power loss.

Explain to me how you are going to find a low resistance drop point with a megger. I have been failed "little" by drop tests. You don't really effin care about proportionate increases or anything else. You are mostly just concerned with an unreasonable drop, and usually the really guilty culprit becomes fairly obvious. This can be helped greatly by "wiggle" tests. Wiggle the key, wiggle the connectors, wiggle any terminals, and just plain shake the living **** out of parts of the harness.

Sometimes it's helpful to add load artificially, I used to have a small carbon pile meant for generator work, not a huge 500-1000+ meant for battery/ starter testing. So if you had a problem in a lighter circuit, like the ignition circuit, you could artificially add "overload" and not really hurt anything--if you are reasonable. Also, in modern times, things I did not previously have, are such things as a temp gun for hunting hot spots