I do not have the 83 diagram but I do have others for the 123s. It seams that MB wired them differently. In one case the horn is wired hot and the horn button goes to ground. The other case is 12 volts goes to the horn button and then to the horns. In eithe case I would start by checking the fuse. The fuse may look ok but old fuses have a tendency to corrode at the ends and not work.

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2009 CLS550 55,000
2004 ML350 144,000
2004 X5 95,000
2002 X5 165,000
1996 320S 155,000 (sold)
1991 190E 192,000 (sold)
1989 300SE 160,00 (sold)
1984 300D 210,000 owned since 85, (sold)
1984 300SD 160,000 (sold)