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This trial has me reconsidering my original plan to tap into existing wiring. Instead I'm thinking about using the add-a-fuse types of things. In the past I've avoided those because I very much want to keep the fusebox cover snapped on to the fusebox so it isn't lost. But at this point I think I'd prefer needing to find a way to store the fusebox cover over cutting into the car wiring.

So my new questions are:
1) I've come across a couple of questions in these forums about using unused slots in the fusebox, but did not find any answers. I'm going to search for that information some more. (Question is, how might I be able to tap into the unused slots in the existing fuse box for always-on, and ignition-on power?)
From my experience and frustration, empty fuse slots in vehicles made by Toyota including Lexus usually do not have wires leading to them or exiting from them. I found that out when installing features not available in the U.S. but which were available in Canada and/or Europe. Maybe there are exceptions.

I've used "add-a-circuit" adapters only in engine compartment fuse boxes where there's no concern with clearance with the fuse box covers.

Another option is to tap into the ignition switch which has both constant and ignition-on power. I haven't done this myself but I watched an installer do it 20 years ago as the final step of a phone system install I did in a 2000 Lexus LS400.

A decade before that, a phone system installer inserted a metal "spade" with an attached wire and in-line fuse holder into the positive side of one of the fuses in order to power a phone system from the under-dash fuse box in a different Lexus. This method allowed the fuse box cover to snap on without modifying it. Professional installers have all sorts of tricks and products.

Or for robust constant power, there's always the accessory nut on the battery positive cable clamp. It's quite easy to pull wires through the big grommet in the firewall above the gas pedal - I did that for both my rear fog light and headlight washers. My 2014 Sienna arrived from the factory with a wire already attached to this accessory nut. I assume it is to power the factory installed fog lights but I've never bothered to verify. I'm powering my trailer lights and headlight washers from the same accessory nut.

2) I'm not optimistic about this, but is there any fuse that would only be powered when the dashboard lights are on? My switches are illuminated and if I could power the lights only when the rest of the dash was lit up that would be ideal.
I provided you with a link in post #5 to a thread that documents how I connected my aftermarket headlight washer switch to an adjacent switch so that it brightens and dims with the rest of the switches.
 

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1) I've come across a couple of questions in these forums about using unused slots in the fusebox, but did not find any answers. I'm going to search for that information some more. (Question is, how might I be able to tap into the unused slots in the existing fuse box for always-on, and ignition-on power?)
You can use a multi-meter or a probing Light to test for the empty Fuse slots. In my experience, the earlier years of a vehicle have them along with redundant wire harnesses. You can use a add-a-fuse (use the one for Toyota's as it has a lower profile fuse connector) to use one of the existing one. For the heaters, you need atleast 20A switched watchout which one you tap. You may be better off simply fishing the wire through the firewall and connecting to the battery

2) I'm not optimistic about this, but is there any fuse that would only be powered when the dashboard lights are on? My switches are illuminated and if I could power the lights only when the rest of the dash was lit up that would be ideal.
Yes, you can tap into one of the wires, I think Grey on my 2012, behind the driver side switch panel
 
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