| I got to thinking last night that there could be significant differences between your 91 and the 90 which is what the online service manual I was using was showing. I found a 91 manual here: https://cardiagn.com/electrical-system-300zx-1991/ It's probably a bit of clickbait site, so just be mindful but it does actually show the manual and there are some differences. It looks like ground is provided to the horn switch through a dedicated black wire in the harness. I didn't have a huge amount of time to look at it, but I think that is actually using the same ground as the cigarette lighter -- probably behind the center console. There also appears to be some interaction with the ASCD -- cruise control system -- and possibly the theft system. It was impossible to tell whether the 'theft warning horn' and the regular horn are the same thing, in which case there are additional relays and systems that the horn wiring may go through.
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