If its like my '75, this is only a warning light for uneven brake pressure front/rear balance, and it does not warn you that the brake has been left on. On newer trucks they added a brake-on warning, but I don't know what year.
The wire for my brake light is on the brake pressure/balance switch, which is mounted under the front of the truck, just under the radiator shroud area. You can follow the lines from the master cylinder down to the pressure switch, but usually need to be under the truck to look up and see the switch.
Bruce