I see you've been a member here for a good many years, but seldom post. You may not know that we have an ELECTRICAL forum where you can download wiring diagrams. Follow this link and download the proper wiring diagram for your truck.
http://67-72chevytrucks.com/vboard/f...splay.php?f=64
It will be a great help in troubleshooting electrical problems. In the meantime, here's some tips to help you get started..
Using a voltmeter or 12V test light, check both sides of the brake light fuse for voltage. A visual inspection isn't the proper fuse test. Sometimes rust accumulates on the fuse box contacts causing bad connections. If you have voltage on both sides of the fuse, disconnect the plug at the brake light switch and check for voltage on the orange wire. If no voltage is present, check the wire for breaks or disconnects. If voltage is present, jumper the orange wire to the white wire. If all circuits are working, you should have brake lights. If the brake lights now work, the switch is either bad or needs adjusting. If the brake lights don't work with the jumper installed, check for continuity of the white wire through the "half moon" turn signal harness connector.. If there is voltage on the turn signal switch side of that connector, you probably have a bad turn signal switch.