69 D-100 brake lights asap!

If I read what you wrote correctly the turnsignals at the rear of the truck work? If yes, then the white wire (S17 / 16WH in your diaghram) going to the turn signal switch you said also had power when you pushed the pedal? You mentioned the power was at the cam on the turn signal switch? I would guess the turnsignal switch is bad. All the rear stop/turn signal power comes out of the turn signal switch!