I am in the process of wiring in a universal trailer light controller into my 2014 Focus ST. In doing so, I've run into a very strange issue which is just beyond my electrical knowledge and I'm guessing it has something to do with CANBus so I've Googled around a bit but nothing seems to match my particular issue.
All the other input pins on the tail light are switched normally, L Turn, R Turn and Tail/Running light are all 0v when off and 12v when switched on (obviously intermittent 12v on the turn signals). There is NO PROBLEM with those three inputs.
The equivalent wire that goes to the brake light bulb, however, is ALWAYS 12v when grounded to the chassis, whether or not the brake is pressed. But yet the bulb correctly turns on and off with the brake.
I feel insane even writing that, but I don't understand how the brake light is switched. Is it a ground switch? I left my voltmeter's positive lead pushed into an open fuse location with +12v and got a complete circuit on the ground side of the brake light socket as well so I don't see how it can be. Can anyone explain what's going on?