Where is the break switch located for rear lights

You should look near the brake pedal, possibly located on or near the pedal arm itself. Your problem may also be the brake light relay. In a vehicle this old the brake light switch may actually be located in the master cylinder or lines, for example, the 1976 MG Midget uses a fluid-pressure brake light switch attached to the master cylinder. Get a T-Bird Restoration Company parts catalog, they should be able to tell you.