[You Need a Thneed]

Before I suited my house, I had a three way switch at the top and bottom of the stairs between the levels in the 4 way split. Those stairs no longer exist, and now I have a switch at the top of the stairs that does nothing. I have bought a wired in nightlight to replace the switch with, and I want to wrap my head around how it needs to be wired. I know that there is still power in the switch (I tested it).

I believe that I will have to add a short jumper neutral wire from the rest of the neutral wires in the box. The red wire will just be abandoned at both ends (I'm quite sure that the electrican capped it at the other end) by capping it.

The big question is figuring out which black wire to hook up, I suppose I just need to figure out which wire comes from the panel by checking which wire has power in it. Perhaps the confusion comes from it being a three gang box with another three way switch and another switch in the same box.

Because there is power to the switch, I suppose that it isn't likely that the power went to the other switch first. However, I suppose that I should maintain power to the other switch as it still is being used in the suite, just a a regular switch, and I imagine that it is still getting it's power from the box in question.

Talking it through i think this is likely how it was originally:

Panel -> 3 way switch in question -> 3 way switch in what is now the suite -> light (or the last two switched around)

After the house was suited, I believe the electrician set it up like this:

Panel -> 3 way switch that does nothing -> 2 way switch -> light

I want to turn it into:

Panel -> nightlight -> 2 way switch -> light

Help me confirm in my mind how I need to do this.