JDM rear fog lights...

I don't have one on my Subaru but do have one on a different car. It is wired from the factory correctly: for use as a fog light. It is intended to help prevent collisions when my car is being followed in heavy fog.

It'd be somewhat tempting to wire it as a brake light if the car didn't already have a third brake light -- but that's not the case for the Subaru.

I'd consider wiring the rear fog as a rear fog, and perhaps wiring a flasher into the central third brake light to increase attention if your local regs permit.

I know you asked how people had them wired and I do not have one, so just an idea. If i were to put one on, I'd run a wire back there from the dash (I'd install a toggle switch) so I could turn it on when I want. Just a little bit of a pita to run a wire but not too bad; I've done it on other cars.