I figured out the problem with my '79 Pontiac Catalina's horn after checking for continuity in the fuse box. The fuse itself was corroded. I replaced it, and the horn sounds again.
While I was in there, I noticed a couple peculiar things. My Number 1 fuse was missing, and the fuse diagram online says a 5 Amp fuse needs to go in there. That fuse is in the line for the dashboard lights. None of the dash lights work except the right turn indicator. Is that bulb wired into the same fuse as the exterior lights? I checked the current through the #1 slot a multimeter, and it fluctuates pretty significantly with the car running, but never goes over 3 amps.
The Number 2 fuse is supposed to be a 20 Amp fuse, but there was a 5 Amp fuse put in that slot instead. The fuse was not blown. The diagram says that fuse runs power to the electric choke, along with the oil pressure switch. I checked the current through that one as well, and it only read around 2.5-3 volts with the car running. Aren't circuits supposed to run at just under the rated capacity of the fuse? It seems to me like that fuse should've popped right away if everything in the line is working correctly, and I'm wondering if this is an indicator that the electric choke isn't functioning because the amperage is so low. I'm not sure what the oil pressure switch is referencing, but the oil pressure idiot light does come on if you put it in run and don't start the car, then goes out when you start the car.
Do you have a wiring diagram to trace all this stuff out? Some local libraries have Mitchell manuals available where you can look this stuff up and photocopy it. You might also want to buy a factory shop manual that has the wiring diagrams. That will give you a lot more information to work with than the fuse diagram.
If your choke opens up reasonably quickly (like within a minute or so) the electric choke is working. You can also test for voltage on the terminal that feeds it with the ignition on. I don't think those draw a heck of a lot of current, certainly not 20 amps. I recently changed the choke on my old station wagon to electric because the choke heat tube had rotted out and the thermostatic spring in the choke housing was weak. It was no problem just piggy-backing onto an existing low-amperage circuit that turns on and off with the ignition.
That's a good idea, I'll see if our library system has one. I found a digital copy of the original GM service manual on CD for $18 and some change on RockAuto and thought about buying that. They also have it in print, but it's a 1978 and 1979 add-on for around $100. The CD has it all in PDF form. I prefer a book over computer stuff, but that's quite a savings going digital. I haven't decided which to get, lol. Which would you consider Looking? Looking at that amperage, I thought something must be wrong, or they really beefed up those circuits, 3 amp circuits blowing a fuse at 20 amps is a heck of a tolerance gap before blowing something up.
I'm cheap so I'd probably go digital and just print out pages as I need them. I have printed manuals for most of the vehicles I've owned over the years but in some cases I do have the info in digital form. A bonus for the digital copy is if it was scanned with OCR you can search in it for what you need.
FWIW I have a gut feeling that you need to check the grounds on those circuits - often a major, hard-to-trace factor in older car's electrics.
I'm relatively surprised how few electrical issues really exist in this car for its age and sitting outside for 10+ years. The big issue is getting behind that dashboard to check out the lights. The cluster needs removed from the front and I don't want to break anything, lol.