The B/W wire for the S4 alternator would be wired to switched voltage but on the S5/S6 alternator that wire goes to constant voltage or the alternator would slowly drain the battery.

And the BTN fuse powers other fuses w/constant voltage.

 

you don't have rear fog light at least ?
I only have the rear one and i have this issue. just checked today, but the fog dash light is even half lit without the headlight on...

 

Previous owner had it wired wrong. I'm all good now.