My wife just pulled into the driveway after returning from a 400-mile
trip to Atlanta and back today. All the way back, she says, the horn
would blow unless the right turn signal was on, and sometimes even
then.

It certainly was blowing constantly when she pulled in.

Any clues as to where to start? I had this happen years ago, but it
turned out that one of the keys on my keychain was cocked into the slot
between the steering wheel and the steering shaft cover, and touching
the steering shaft.

That's not the case this time.

Thanks in advance,

Jim