If a filament of the tail light would break and fall against the filament of the brake light, it would power the brake light, and also back feed the circuit to think the brakes are being applied, not allowing the cc to work. I saw this years ago when a vehicle wouldn't go into lock-up because the ECU thought the brake was being applied. When you apply the brake, the transmission would come out of lock-up so the car wouldn't stall.

James