A good proportion of the problems raised on here are about dificulties in understanding wiring.
I spent a lot of effort, very stupidly, trying to write an explanation of how I had wired a horn-relay on a TR4A.
I forgot that one picture is worth a thousand words. Especially on wiring.
The originator asked where to get a wiring diagram. This isn't in my WSM.
So this evening I drew the circuit on a scrap of paper, in about 30secs and photographed it, with the built in camera. It needed a flip and a few colour adjustments, but was actually really quick to do.
This isn't the only way to wire this. Its how I did mine. There are other ways that will work just as well.
I am really posting ths here to encourage the use of more diagrams for explaining this kind of thing.