I can't read your diagram. You either need to crop it and post full size sections or post a link so we can download it.
I've no idea what a "brake light ground" is. Nothing like that goes to the cluster.
They MIGHT be talking about the switch wire for the brake WARNING lamp. That wire goes two places-----wires to the brake warning switch on the brake distribution/ proportioning valve, and branches off to the switch on the parking brake lever
And as usual, NEVER tell us what you are working on, year????
