steering column to see if another owner may have had an aftermarket alarm, car starter, etc spliced into the car's wiring. The horn would be part of that as either an alarm or confirmation tone. It's possible the splice is bad even if the gear has been removed. An online service manual can be found here: [ http://www.300zx-twinturbo.com/cgi-bin/manual.cgi ] Go to EL section, page 89 for the horn circuit diagram. If no wiring issues under the dash I'd find the harness connector that goes to the steering column and try jumping the horn switch line to ground to verify the harness connection is good --- horn sounds. If good, very possible the connection slip ring in the column isn't working right or needs to be cleaned - no idea what you'd need to do there tho?
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