Brake light, electrical problem

I have an Impala 9C1 version. The brake lights have worked on occasion, mostly they don't. The turn signals and emergency flashers work ok (same filaments). The brake switch at the pedal has voltage and works when checked with VOM, and jumpering the 2 pin brake connector does not make brakes lights come on. The car has a surveillance mode which is kept in the disabled (off) state. Not sure where to look, or how to get a circuit diagram to trace the problem.