Magnetic Trailer Surge Brake Lockout Key for UFP A-60 A-70 A-75 A-84 Actuators, Replaces 34557, Heavy Duty Easy Release Strap, No Tape No C-Clamps

IF the surge brakes have a lockout solenoid, that blue wire triggers it when the trucks reverse lights come on. Not all surge brake units have that function and it required a 5 flat or more style connector. You can also buy a magnet thing that goes in the slide mechanism and locks it out mechanically. Amazon example