1980 Chevrolet Tahoe tail lights

You have multiple problems so don't suspect the flasher. That isn't in the running light circuit. Look for what they all have in common. That's the ground wire. If signal lamp current can't get through the rear lights to ground, there won't be enough current flow to heat up the element in the flasher, so it won't flash. Depending on the style of flasher, the lights will either stay on or stay off when the signal switch is turned on.

To verify a bad ground wire, turn on any of the rear lights, then measure the voltage on the brass base of a bulb. There should be no voltage there.

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.

Forgot to mention that for this type of problem, a test light can be a lot more accurate than a digital voltmeter. All it takes for a voltmeter to detect a voltage is a little speck of carbon between the ends of a broken wire. Enough current to run a test light won't get through that carbon, so the test light will correctly show a lack of voltage, just like a bulb will see.

That sounds like you found a valuable clue. There's either a break in the ground wire for the rear lights or there's a ground strap between the box and the frame that's rusted off. Boxes are usually bolted right to the frame with no rubber isolators, so a ground strap isn't necessary. You might want to verify your findings by doing a continuity test between the brass socket ground and the frame. You can also use a temporary wire to touch to the frame on one end and the ground housing on one of the sockets.

The goal of a new wire is to replace an old one that has a lot of corrosion. If you see a copper wire that is dark and dull brown, that's because it's corroded. Solder will not adhere to it no matter how well you shine up the strands. For that reason you have to cut it back far enough to expose nice shiny copper, and splice it there.

If the original attaching point is rusty, you can drill a new hole, grind the rust or paint off, then run in a new self-tapping screw to attach a new ground wire. How you do it and where you splice it depends on how close you can get to clean and solid material to attach to. If you're splicing a new wire to a part of the old wire, slide the strands together, (don't twist them like we do with wire nuts for house wiring), solder them, then slide on a piece of heat-shrink tubing to seal the connection. Don't use electrical tape because it will unravel into a gooey mess on a hot day. Auto parts stores have heat-shrink tubing with hot-melt glue inside to seal splices that will be exposed to moisture.

At the light sockets, I'd look for the ground wire and attach the new wire to it. As long as the break is before that point, all the other lights will have their ground wires spliced together, so your new wire will take care of all of them. The exception would be if a different ground wire was used on each side of the truck. If that's the case, you'll only have working lights on one side, and you'll have to do the same repair to the other side.

Please clarify this: "The brake lights I believe are stuck on are there not working not sure which it is any suggestions on that." Do you have brake lights when you press the pedal, or are they on all the time? If they're on all the time, look at the switch and be sure the pedal is pushing the plunger to turn it off.

If you DO have brake lights, you should have rear signals too. The brake light circuits goes through the signal switch. They use the same wires and filaments in the bulbs. Only the running lights are separate. They use the smaller and dimmer filaments. Check that the correct filament is lighting up. The brake lights use the brighter filaments.

Each rear brake light circuit goes through half of the signal switch, but only when the signal switch is off. When you activate the signal switch, it breaks the connection to the brake light circuit and makes the connection to the signal flasher. At the same time it connects the front light to that circuit. The same (brighter) filament is used for the brake and signal systems. On the front, be sure the brighter filament is the one lighting up for the signals too.