One of my friends has a 2007 Lincoln MKX that's been having some electrical issues lately, the battery drains to dead overnight. It's been looked at by several people (including dealership), the first few of who pretty much had no idea, and suggested basic things like replacing the battery, which was done but to no avail.
I wanted to look at it briefly and actually get to the core of the issue instead of speculating, so I bought a 100A current shunt and voltage/current readout, hooked it in, and started to check it out. The battery was sourcing 1.4A with everything off. I started going through the fuses, and found a few that caused the current reading to drop, but ultimately I didn't have enough time on my hands due to other obligations to go through and definitively find the problem before my friend needed this car working again.
Instead, it was taken to another friend who is pretty smart and works on vehicular electronics. He says he found the issue fairly quickly. I heard 2nd hand that it was either corroded or incorrectly soldered terminals/terminal wire.
I don't know this friend well, but I know he is pretty smart. Still, it struck me as odd. I would have thought corroded terminals or incorrectly soldered battery cables would cause an open circuit if anything, not parasitic current drain. But perhaps I don't know electricity as well as I thought I did (degree is in computer engineering, so I've done a fair bit of EE, but not automotive electrical).
So how could corroded terminals or incorrectly soldered battery cable cause the behavior I saw? Even so, how would that explain the current drop I saw on multiple fuses? I saw 0.5A drop pulling the radio fuse out, 0.5A on the cluster fuse, and smaller drops on a few other fuses.
Car is dead again, as I suspected it would be. Never left the driveway this morning. Guy who "fixed" it is very smart, but the family member who dropped it off with him gave him no context, no background, no mention of what I found, probably didnt even tell him the battery kept dying overnight. No clue why, but now we might actually get somewhere. I got in direct touch with him and told him everything. Thanks for confirming my suspicions.