The early main harness only had wires for the single horn on the off-side:

For the optional second horn a sub-harness was needed, and the original horns had double-spades like these BHA4514 and BHA4515 horns from Motaclan/Leacy:

But if you have the later single-spade per terminal horns you can avoid mangling the harnesses by using a male-male-female spade adapter. Loads in America apparently, but I've not been able to find any in the UK, or even China (which is surely where they come from). I'm not paying the ludicrous postal charges that seem to be the norm from America, so looked at making some up. It occurred to me that I could use a piggy-back (one male and one female) and a male spade, back to back. Conveniently my piggy-backs have an second tube around the main crimp tube, and extending back from it, so with the insulation stripped off both the back of the male slots neatly into that. A copper nail that extends through both components, crimped, soldered and the whole thing heat-shrunk makes a neat job:

Subsequently I came across these via Google. 6.3mm from several sources - UK, two in America, in side-by-side or back-to-back configuration, another here in side-by-side configuration, and one in Australia. However the 'side-by-side' type look as if the spades are too close to each other to fit two insulated females:

In January the bracket changed to angled GCE110 mounting the horn(s) in the inner wing(s). Two horns became standard in October 69 at the start of the 1970 model year: (Motaclan/Leacy)

Mixo horns at least are mirror-images:

Mounted vertically with the trumpet facing forwards and the spades uppermost ...

... the curve of the trumpet is downwards (as indicated by the arrow), so anything that gets inside will lodge there:

This is what came out of just one of mine (not the tape measure, that's just to indicate the scale ...):

They would need to be mounted such that the curve of the trumpet is upwards (see arrows), which will resist water and debris getting stuck inside, even though this puts the spade connections at the bottom. This can be partly alleviated by angling the open end of the trumpet downwards to some extent, which will further help avoid muck getting inside:

Bee came to me with them mounted direct to the inner wing and not on brackets, I've left then that way, but orientated to resist dirt and water going into the trumpet. Connections now underneath, but still accessible by touch:

At some point - possibly with rubber bumpers - each off-side connection gained a rubber boot with two separate holes for the wires, possibly to protect the connections from the worst of the weather:

However going by Vee at least the near-side didn't. Both the Mixo horns on the roadster and the different make on Vee have the spades to one side which means they can be mounted so the trumpets face across the car and avoid the worst of the weather driving into the trumpet. Clausager shows a 72 model (p65) like this, but bear in mind the horns in his pictures may not be original, as mine aren't:

This drawing from the 77 model year and later Leyland Parts Catalogue shows the spades on the opposite side to the trumpet, which means the trumpet has to be facing forwards (or backwards) or the spades foul the bracket. Clausager shows a 69 model (p63) with them pointing forwards like this: