First, make sure the bed and rear lamps are properly grounded. Next, physically remove the brake lamp fuse to inspect the fuse socket for corrosion (a common problem). Make sure the metal end caps of the fuse are not loose on the glass tube and that the fuse element is not cracked. Once the fuse and fusebox are physically verified, check for power to and through the brake light switch with the brake pedal depressed. Brake circuit power passes from the brake lamp switch up the steering column into the turn signal switch, so validate the brake lamp circuit connection through the turn switch harness connector along the right side of the column. If brake lamp power is available to this point, replace the turn signal switch.