Everyone:

This is not quite the final report, but I have found a series of problems.

First, the plates visible through one of the battery fill holes was higher than the others, so I suspect it was swollen and possibly shorted. The battery was at least 7 yrs old, so I replaced it.

I hand-over-handed the alternator field and charging leads and except for some heat embrittled conduit didn't find any insulation breaches.

The horn went out when it's fuse burned. The fuse burned because the wiper motor failed - strong odor when I began to disassemble. By its looks it could be an original (206Kmis). I will replace tomorrow.

I will get a Bently manual and do some electrical health and welfare checks to be sure no more gremlins are hiding.

Thanks to Aye Roll, KarmaticLove, Dick in Falls Church, and Jim Holst for all your help.