[KimMG]

Some vehicles have wear sensors in the brake pads that will cause the light to come on. Need to make sure the float in the master cylinder floats and is not full of fluid and sinking.
How is the truck braking? Does it pull? A failing master cylinder can cause the light to come on.
If the problem is electrical, then follow the circuit and find the wire that is grounding. Look for a pinched wire or a wire with worn insulation. A wiring diagram will help guide you through the circuit. Most likely the short will be after the bulb in the dash.