Why Does Ground Wire of Brake Controller Need to Run to Battery when 7-Way Grounds

Question:

Very confused with the video for wiring brakes to our subaru outback. There is a white wire that is grounded at back of vehicle. The white wire inside the grey wire: is this grounded to the engine bay as the video shows? If so, where does the other white wire come from that goes into the car through grommet?

asked by: Dave G

Expert Reply:

I am not totally sure which wires you are talking about, but I am assuming it's part of the bonded wire that runs up to the engine compartment? If so, the white wire gets grounded as it's part of wiring up the brake controller which is different than the ground at the 7-way which is for the ground of the voltage that the controller puts out (among other circuits.)

I attached a diagram that shows how everything wires up as well.

expert reply by: Jameson C