I think the 'dot inside the loom' you're referring to is simply a permanent connection.. think of it as a blob of solder!
On these old Hondas, and according to my wiriing diagrams:
A RED wire is permanently live (+ve), ie 'hot wired' to the battery.
A BLACK wire is switched live, ie only +ve when you turn the ignition on.
A GREEN wire is permanently grounded (-ve). Ie at all times connected to the -ve terminal of the battery, via the frame.
A LIGHT GREEN wire is switched earth, ie only connected to the frame when you press the horn button.
So, using my diagrams. One side of the horn has a black wire (ie +ve when ignition on) and the other side of the horn has a light green wire.... which connects to earth when you press the button, completes the circuit & sounds the horn.
BUT. A change of switchgear can nullify the above, eg if off a later model / different CB. In which case the above circuit may not be quite correct.
Bottom line simply...
Connect a switched live (+ve) ... should be a black wire... to one side of the horn
Connect the light green to the other. If it doesn't work then no harm should be done.
If this doesn't work your set-up / switchgear is likely non-OEM and then you should try..
Connect one side of the horn to (permanent) earth (by the book should be a green wire to the frame!) and the other to the light green. If it works great, if it doesn't then no harm will be done as you'd be simply putting two earths across the horn.
(Comment: NEVER simply connect an unknown loom wire directly to +ve or -ve, ie always have 'something' in between, such as a horn / lightbulb.... or best of all, a multimeter! (See below)
A fantastic tool, and from £5 upto £500 is a good old multimeter. Will save an awful lot of time / tears. Typically you can...
Test for continuity... ie is this end of the wire connected to this wire at the other end of the loom? Or is this wire earthed? Buy a meter with audible beep to make this job a breeze.
Test for voltage... Eg is this wire perm live, or switched live? Connect one lead to your wire, the other to the frame & check to see if 12V permanently ... or only with ignition.
Test for voltage regulation.... Put the leads onto the battery & rev the bike... read the voltages against RPM & check within spec.
Test for current consumptions, battery drain... 'Break' the circuit and put the meter leads in between.. and read the amps flowing. (many meters only do milliamperes, choose one that can read upto 10A.
The list is endless... and it ain't as daunting as it all might seem.
First one I found on ebay...
http://www.ebay.co.uk/itm/Digital-LCD-Multimeter-Voltmeter-Ammeter-AC-DC-OHM-Volt-Tester-Test-Current-/160994004691?pt=UK_BOI_Electrical_Test_Measurement_Equipment_ET&hash=item257bfd8ed3Good luck,
Simon