I too can take a long time to reply. I had a major house fire and have to sit in front of the public library to get on the internet, so my replies are going to be sporadic.
Lets start with the simplest circuit so we can eliminate flashers and signal and brake light switches. We know the running light circuit is okay up front so the headlight switch is working. That leaves the wiring to the rear, and those sockets. Remove all of the bulbs in the rear except one tail / brake light bulb. That's because if there's a bad ground, current can still go through a bulb, can't get to ground, but it can find an alternate path to ground through a bulb on the other side.
If that one bulb doesn't work for the brake light or the tail light, remove it, then turn the tail lights on or have a helper press the brake pedal to turn the brake light circuit on. Measure the voltage on the two small contacts inside the socket. You should find voltage on only one of them. If you do not, there's a break in the wire between there and the bulkhead connector on the firewall. Given that you have multiple symptoms and multiple dead circuits, a good first suspect would be a wire harness that got melted where it goes too close to the exhaust system, or someone installed a trailer wiring harness with Scotch-Lok connectors and they all corroded. Those quick connectors don't seal out moisture. Also, in that time period GM was using a lot of aluminum wires, and if there's any nick or cut in the insulation, it's only a matter of a few weeks before that wire will corrode through.
If you can't see any sign of damage to the harness along the frame rail or under the box, you'll have to take the harness apart to find the break in the wire. I'm betting you're going to find a damaged area and all the affected wires will be broken in the same place.
Monday, May 5th, 2014 AT 7:53 PM