First thing I would do is make sure the brake light switch is depressed when the brake is OFF, pedal is in the all the way back at rest position. Check the wires going to the brake light switch with a test light. The key doesn't have to be on & 1 wire on the switch should have power all the time, the other wire should have power when the pedal/switch is activated, as in switch plunger is moving out (Getting longer) or pedal is depressed. If you have power going in & don't have power coming out when the pedal is pushed, the switch is bad. I don't think the flasher will have anything to do with the brake light operation.