Headlights work, nothing else

[zac]

‘79 K20, just downloaded Hatzie’s wiring diagrams. Don’t know what I’d do without him. While I’m going thru the diagrams maybe somebody wants to take a guess. At Home Depot all the lights were working. 10 minutes later at Lowe’s I only had headlights, brake lights and a dome light. Tail lights, license plate lights, clearance, marker and instrument lights out. The last one was weird, thought it had a different fuse. Got me thinking it could be the light switch, but it seems ok. The taillight fuse was blown and sort of melted. Looks like the terminals in the fuse block are burned.

[Dead Parrot]

Pretty sure the 'running' lights & dash lights are on the same fuse. The Brake/turn lights are on a different fuse. Also the bright headlight indicator might be on a separate fuse from the dash lights. I had a 68 Impala with your symptoms. Made a 100 mile drive in the dark thinking only my dash lights were out(didn't have a replacement fuse). Didn't realize until I began troubleshooting the next morning that the running lights were also out. Like yours, my dome light worked.

[cadillac_al]

Tail lights and instrument lights are almost always on the same circuit. It must be a fuse, and maybe something blowing the fuse.

[zac]

It appears I had two issues. The wire going to the clearance lights was fouled around the a/c duct and was bare and almost broken. It snapped once and for all will I was disassembling and it was hard to figure out because what was left disappeared into the door pillar where it goes up. Also, the diagram just designates it is a taillight terminal, but it’s not the one that actually goes to the taillights. Most references say it’s blank in the harness. I’m pointing to the clearance light terminal in the picture. You can see it’s ganged to the actual taillight feed.

Messing around with my meter and the various circuits from the switch, it appeared I had a short in the actual taillight harness in back. Go figure, the truck has had a hard life and the usual poor attempts at trailer light wiring, so I’m replacing all that.

I spent a lot of time with the diagram and a multimeter. There’s more going on with the switch than I thought. The taillights have their own feed and fuse, the instruments have their own fuse, and the headlights have a separate feed and internal circuit breaker. I think it paid to take some time and sort through it rather than just randomly replacing stuff. My switch works perfectly.

[Accelo]

"I think it paid to take some time and sort through"
Almost always the case.

It is extremely difficult to do troubleshooting on the internet.
Especially if the person, with the issue, has little or no knowledge of electrical circuits and the tools needed to track issues down.
We all hope for the simple common solution, these are few and far between.
You had both the tools and diagrams needed. Throwing parts at it almost never works.
Congratulations on getting after it.
Cheers.