[JimKing]

Are there any terminals actually at that position in your USDM fuse box? I would strongly suspect the USDM dash harness lacks any wiring for RHD UK only mirror heaters.

One terminal (for the common power buss) may be present, while the other side is empty. If you are industrious enough, you can scavenge fuse terminals from a donor Toyota fuse box (almost any 90s car) and add them to your fuse box and run custom wiring from there to your gauge.

Although looking at the fuse box illustrations above, it looks like the lower right position doesn't share a "power buss" with any other fuses.

 

[JimKing]

That's interesting / encouraging if there's a fuse in that location. Then I'd expect the terminals to be there as well.
In a similar vein, when it came to USDM cars and fog lights, the fuse box terminals (for the relay and fuse) were present in only some of the early turbos that were delivered w/o fog lights. Cars with EHPS and/or ABS got a "loaded" frunk harness that had this wiring (including the fog light connectors themselves). Retrofitting fog lights on these cars was just a matter in mounting them and plugging in the relay, fuse and cabin switch.

But "stripper" turbos had none of fuse box terminals, wiring or connectors in the harness. I had one of those cars.

 

[JimKing]

That doesn't match the 93 USDM diagram posted above. Lots of things are in different positions. Could it be that the previous owner of your car lost the this cover, and (accidentally) bought a UK or JDM one to replace it?

Is your RAD CIG fuse really in the lower left position??