Which unused fuse location(s) is always on?

I have no idea. If I did want to know then I would use a test light or multimeter to find out which ones have constant battery power. You have a specific project that needs an ignition-off feed?

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You can split an existing circuit easily enough with one of these

 

For future work on the car go on eBay you can pick up the Toyota factory electronic manual pretty cheap like $30. it has complete schematics of all the cars circuits.

Personally I would stay out of the cabin fuse block it’s in a very hard position to work on.
I would just bolt a fuse pig tail on the main front jump point with an inline fuse.

That what I did to install a air compressor for air horns. They were incredibly loud I loved them.

 

When you look in the electrical wiring diagram (more info), you will find a section on relay and junction blocks, which will show their outer appearance and also their inner circuits, many pages in this vein (example only, this is from a Gen 3):



That was taken from this post, which in turn took it from this even more detailed post.

 

Probably the same engineers that designed the rear hatch inside release that’s nearly impossible to get to when you need it. Idiots.

 

The fuses are on the bottom face of an important junction block between a few different wire harnesses, with key relays and the body ECU also contained in it. Building it like that neatly solves a bunch of wiring challenges in one swell foop, and lets the car be very quickly put together at the factory.

The bottom face of that thing is the most accessible. The other faces are all taken up with wire harness connections or the ECU, and it takes some dash disassembly to reach them. Some of those connections face the firewall.

It did have a nicer location in Gen 1. It lived at the very left end of the dash, and you could access the fuses through a cover you could remove when the front left door was open. I'm not sure why they moved away from that design. Maybe the other connections on it were even harder to reach when it was there.

 

One pin in the OBD port has constant 12 volts FWIW.