dealing with a 1994 Thomas/ international bus . t444 engine. issue is when lights are off both brake lights work properly , when the head lights are on the drivers side rear brake light shuts off when the brakes are applied , electrical is not my strong point . I've checked the wires , the tail light assembly and the tail light switches on the brake pedal. I know that there is something that I am missing but I am stuck right now. any suggestions would be appreciated