Yeah it’s big bummer that Toyota computer logic can’t see passed it.
To answer your question on brake controller operation, they have inertia sensors so really the pedal application input is just on or off. The force with which the pedal is applied is calculated via the inertia sensor in the brake controller. The settings I would guess range from 0 volts to charging voltage (13-14volts?) In my trailer set up I run Redarc at about 4 on the dial so what’s that like 40% give or take of max? So on light brake application (less extreme inertia/momentum shift present) the brake controller is going to apply UP TO that 40% mark. So even less than the 5 or so volts.
But all we’re concerned with is manual override which does the full value of which you have it set. So the 40% in my case during manual override. During normal braking the brake lights operate just fine remember. We’re ONLY concerned with manual override having the issues.
Therefore it’s unlikely to ever be much less than the 5 or so volts for most folks with these 4runners. But it could be higher I would guess.
Meaning even further, your 6 volt relay idea would probably work just fine.
The idea I came up with used a seperate feed line off of the back-feed principle through the trigger line stated earlier since we know the brake input is a 12v feed we could use for a relay
I say “magic box” because on a trailer the turn/brake is the same light but on the 4runner they’re seperate. Somewhere they combine. On my 95 4Runner there’s a magic box you buy to install between tail lights and trailer plug that combines it all.
Basically you mimmick the feed source to the plug instead of the output source. Still need 3 diodes and a relay and run of wire to and from the 7 way plug and brake controller. Pain in the ass regardless.