Wow, Thomas, you have outdone yourself again. I am greatly impressed, as I am sure we all are.
I thought it was JOHN Lucas, Prince of Darkness. (Rather than Joseph) Isn't it the wiring and the connectors, rather than the actual devices that failed on the old British cars? I can only remark on one MG-TD and one Jaguar XK-120 I worked on many years ago, when in high school. The ground seemed to be the biggest problem. I seem to recall working on something with a positive ground, too, years ago.
My 300TD had two horns. One was the stock and one was a replacement. Together they sounded like the wailings of a dying shrew. I replaced both of them. Now it sounds great, although I still long for the sound of an 18-wheeler.
I assume that this would require a compressor and some relays.
Semibodacious Transmogrifications a Specialty
1990 300D 2.5 Turbo sedan 171K (Rudolf)
1985 300D Turbo TD Wagon 219K (Remuda)
"Time flies like and arrow, yet fruit flies like a banana"