My horn stopped working one cold morning, then revived for a few seconds
and has now stopped entirely.

I tested the two horn wires with a voltmeter and since pressing the horn
button caused 12V to appear on one of the wires I replaced the horn with a
new one for GB#5.88.

However this did not do the trick. The button works fine. Both old and new
horns work fine (when connected across the battery). Fuse number 12 blew
during testing but I replaced it and the horn still did not work. The only
way I can get the horn to work is by plugging the positive wire into one
terminal on the horn unit and then grounding the other terminal onto the
car.

Can anyone explain what is going on?

1984 Caravelle GL auto 190k miles.
Lionel Tun
ark@saqnet.co.uk