Good Method to Add Wiring For Add-Ons

That circuit is a little different as it should be getting 12v through the fuse (which you have verified is good) and it’s grounded when the brake light switch moves off center inside the proportioning valve.
My guess would be the 12v comes from the circuit board behind the instrument panel, through the brake light on the dash, through the fuse and out cavity M to the switch to be grounded.
Could you have been pulling 12v from the instrument panel, through the bulb and the bulb is burned out? That would cause an open in the brake circuit when everything else is working.