Wow, good find, I never would have thought to check there. Are you sure that isn't just the parking brake switch tho? If it is that wouldn't be the right wire to use for a trailer brake controller. I know on my truck there must be a switch on that parking brake air switch because when I release the parking brakes the daytime running lights come on.
'03 Freightliner FL112, 295" wheel base, with '03 United Specialties 26' living quarters, single screw, Cat C12 430 h/p 1650 torque, Eaton 10speed , 3.42 rear axle ratio