it will turn out to be one of these two things: .

1) shorted wires in a bundle: it's intermittent because it's signal level amperage so not enough to blow fuses, melt wires or weld together (to stay shorted) or
2) some device that shares common pins has an internal fault that causes back feeding of power onto an input line. Intermittent because it's low level signal but the fault trips some version of a "holding relay".

I would look for any non mission critical devices and unplug them. Half at a time if possible; divide and conquer.

I was also thinking those modules which have both continuous (Terminal 30) and switched (Terminal 15) supply. But still something doesn't add up. When this happens everything seems to work normally so there is quite high amperage needed. Both Terminal R and Terminal 15 rails need to be backfired and as measured they both are and have battery voltage so the source can't be any thin wire or low amp fuse. And this doesn't explain why at the same time those supply lines for ignition switch drop voltage even though they are connected to front fuse box with 4 mm2 wire and 50A fuse which just happens to look like it's been hot by the colour change of the connection on fuse box.

A ground terminal off somewhere...?

Just as a quick check, I unplugged every plug on the top of the fuse box and inspected the plug and wires of each. I didn't see any discoloring of any wires, pins, or plugs. I know that's not much of a check, but just a reference point.

That sure seems like a lot of voltage going back through some very small wires and that worries me that every time I'm doing this, it's possibly frying things upstream.

Any suggestions on what to start unplugging? What about starting to unplug fuses instead of hunting down devices?

It may very well be a short. So far, and I'm not done yet, I haven't found a single thing that looked out of place or wrong. Everywhere wire comes through a hole, there's a rubber grommet that's in tact. When I look at the wires around it, there's no signs of wear. I will confess, the amount of wiring behind the dash is overwhelming. Looks like a space shuttle. Massive bundles of wires, all still perfectly bound together. It has to be 100's and 100's of wires. If it's a short in one of those bundles, I seriously doubt I'll ever find it unless my diagnostic skills or tools improve.

What about a bad relay as an option? In my mind, I'm thinking if the relay is stuck in the wrong position, then it's allowing voltage through when it shouldn't be. Would that be an option? If so, any suggestions on which relay?

By the I'm not sure if you have already mentioned it. When this happens and you disconnect battery, what if you connect the battery back right away, what happens? Is everything back to normal or still powered?

It can go either way. Some times it's fine, but most of the time, you have to leave it unplugged for a bit for it to "reset" things to where seems like it's operating normally.

The problem is the intermittent nature but: when you get it to stay on:

Pull the key and then (duh), pull fuses one at a time until the car shuts off.

That should eliminate a bunch of possibilities. There may be more than one device involved (faulty device shared line with another where the fuse pull breaks the connection to hot).

Open ground can easily put weird voltages in weird places so also do some ground wire testing.

Can you point me in the right direction on where to start with that? Everything I know about electronics has been learned in this thread :)

Search xo. Somebody posted a step by step.