There is no way for me to know.
My A35 schematic shows that BROWN is used for grounds.
It also shows that BLACK is for the ignition circuit. It shows two black wires:
- one black wire goes to the engine shut off switch; and
- the other black wire goes to the ignition coil.
Sorry - can't help much with this particular question. You could always just track the wire back to its actual source - and see what it is for.