The part about flexing 20 year old pins is the real issue here. Those cannon plugs have been sitting in one position for two decades, the wire bundles have taken a set, the pins are probably already got a little corrosion or fretting on them. When you grab that harness and yank it around you are moving stuff that was never meant to move like that, and yeah you might make the fault show up, but now you have also put stress on every other pin in that plug that was fine. I have seen guys wiggle a connector and turn one intermittent into three, and then you are chasing ghosts for a week. The Fluke on the ground tells you what the circuit looks like in the condition it lives in, which is bolted down and not getting yanked on. If you want to test for vibration, get the plane running or use a proper shake rig, don't just grab and twist. Wiggle test catches the fault your wiggle created, not the one the plane actually has.

My buddy runs a bike repair shop and had the same thing happen, his hours were wrong for like 8 months and he never touched the photos. He spent 20 minutes fixing it all one night and started getting calls the next week from people who said they found him on Maps. It's wild how much the basics matter when nobody ever bothers to check them.