Power for the brake lights comes from the fuse box to the brake light switch at the brake pedal, then on to the harness connector on the steering column. From there it flows to the signal/hazard switch and out to the brake lamps. If the rear signal lights work the circuitry will be good from the signal light switch rearward. Check if you have 12 volts going to the brake switch, if yes check if you have 12 volts leaving the switch when the switch is closed. The wire going to the brake switch from the fuse box should be green, in a 68 car, and the wire leaving the brake switch going to the connector on the steering column should be white. If you have voltage entering and leaving the brake switch the problem could be a break in the wire from the switch to the harness connector, the harness connector itself or the signal/hazard switch.