how does a brake light switch keep a 1990 ford thunderbird from starting

Because the brake pedal has got to be depressed before the starter will engage .. Therefore IF the brake pedal - starter interlock switch has failed .. The starter won't CRANK the engine .. IF you've owned the car for awhile, surely you have noticed that unless the brake pedal is pushed .. The engine will NOT crank... Now there are yet more possibilities for the no crank condition.