I have a 1991 740 wagon with a horn problem.

It still had the original Bosch horns when I got the car in 2021, so they lasted about 30 years. One of them failed, then the other failed. I took them off and it wasn't a surprise because they were so corroded.

Tl;dr: I've done way too many horn replacements and they keep burning out. I have hooked my multimeter up to the connections and there's a good steady 12V when I press the horn button with the ignition on.

I got a Wolo horn kit from AutoZone, and it worked for awhile... about a couple of months. I found a real cheap horn kit with two horns at Harbor Freight Tools. I installed that and they worked for a couple of months. I replaced those with one of AutoZone's SureBuilt horns. I lasted a month or so through the tough winter. That one got some water damage. I got my mechanic to replace the horn during an oil change in April. He put in a single Wolo horn. It lasted about six weeks. I bought a pair of pair of Fiamm Freeway Blasters on Amazon. I hooked them up, and the very first day they were intermittent and sounded like sick ducks. After sounding them a couple of times they were both burned out. I got another SureBuilt disk horn from AutoZone and kind of liked it because it sounded a lot like the original Bosch horns. Too bad that it only worked for a few weeks. I got another one to replace it and it burned out almost immediately.

Why do horns keep failing? Does anyone know what's going on? It's almost like they sometimes burn out almost immediately. Other times, if I'm lucky, I get a few weeks to a month or so, but they always burn out. The fuse in the car has never burned out! How do I troubleshoot this? I'm at my wit's end!

 

I'm not sure what your issue is, however I wired up a separate horn relay. As I understand it, the horn circuit runs through the buttons and aftermarket horns draw more amperage than the stock pancake horns.
(I'm not an electrical expert, by any means)

 

Good job, admasters!

After I posted the question a couple of months ago, I talked with a mechanic who worked on this. He told me that they suspected that it all came down to the "pads" in the steering wheel that are part of the switch. I've seen other posts that say that's the problem. In my case, I was getting a good stable 12V from the original wires to the horn when I pressed the horn button. I hooked up the wires that went to the original horn to a relay. It switches perfectly. On the other side, I switched the 12V from the other original horn wire to the new horn. I grounded the new horn to a bolt on the body. Both horn buttons work fine. The horn works reliably now, and hasn't burned out.

I've got some louder horns that will definitely take more amperage, so when I hook those up, I'll run a wire directly to the battery for 12V and fuse it with an inline fuse.