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Thanks for the great information. That seems to be the problem. With your lead, I did some investigation using a troubleshooting chart I stumbled across in the service manual. It said to disconnect the overheat sensor and turn ignition key to “ON” position. If the horn still sounds, then the warning module is faulty. If horn does not sound, then engine overheat sensor is faulty. The horn kept sounding. Did all other tests called for in the chart and they came back negative.

Took your suggestion and found a Mercury aftermarket sensor on eBay for $159:
https://www.ebay.com/itm/293964709835...R-iSicvdYA

Have you, or anybody else on the forum, ever heard of Arieltec and/or used their products?  On my engine, the module was stamped with part #41470A23 but that part doesn't show up anywhere (see attached photo), although the part you quoted (#41470A26) looked identical. Also the wire colors on the Arieltec aftermarket don't exactly look the same and I don't see the tan with blue stripe wire. Any additional thoughts would be appreciated. Again, many thanks for helping with my issue.