See horn wiring diagram here.
Be careful and do not burn up the wiring harness.

Through all of this, a test light is your friend.
Ground the test light, and touch it anywhere.
When it lights up you have power. No light equals no power.
If it lights up dimly, then you have reduced voltage.
Always check the test light first on a known good power source to be sure the light works.
Use the test light frequently, any time you do not know for sure if power is present or not.

Do the following in the listed sequence.

Test the horn first
Disconnect the two wires at the horn.
Grab two jumper wires, and connect power and ground directly to the horn.
If you get a big spark with final connection, and it does not work, then the horn has an internal short to ground.
If no spark, and the horn does not work or sound at all, then there is open circuit inside the horn.
That might be dirty contact points inside, that may be cleaned if you remove the cover.
If the horn works, but not loud enough, then try adjusting the set screw with the jam nut on it.
If it does not work, then fix or replace the horn first, before you waste a lot of time chasing the wiring that may be perfectly good.

When the horn works, then the problem will be in the wiring. Then more testing.

Checking the button wiring:
Connect the hot jumper wire to the horn.
Also connect the Brown /Black return wire in the harness to the horn.
The working horn will serve as a current limiter to avoid burning wiring.

If the horn works when you press the button, then the button wiring is good.
If not, then find and fix the open circuit fault in the button circuit, which is switching the horn to ground. That might be disconnected or dirty wire connections or failed button contacts.
It is unlikely to be a burned wire with open circuit.

Open circuit faults are almost always at the wire ends, very seldom open circuit in the wire inside the harness. If it ever is open circuit in a harness wire, then the harness is likely fried from over-current from having shorted power to ground. That requires cutting into the harness to repair or replace the disconnected wire(s), or replace the harness.

If the horn blows when you connect power jumper and the switched wire, without pressing the button, then there is a short to ground in the button circuit. Find that fault and fix it before you proceed.
If the horn does not blow when you connect power jumper and he switched wire and press the button, then there is a open circuit or failed switch in the button circuit. Find that fault and fix it before you proceed.
If the horn blows when you connect power jumper and the switched wire and press the button, then
the button circuit is fine,and the fault will be in the power feed circuit between fuse box and horn.

Checking horn power circuit:
Disconnect horn. Connect a jumper wire from horn to ground.
Connect the Brown/Green harness wire to horn.
You should get a small spark, and the horn should blow.

If that works, remove the fuse,and the horn should stop, replace the fuse, and the horn should blow. If the sound is weak or intermittent, clean the fuse contacts and check the wire connections to the fuse box. Also check the power feed wire from control box terminal A. If still a problem, then replace the fuse box and/or fuse.

If horn is grounded,and the harness power wire is connected, and the horn does not blow,
then check to see why there is no power from the fuse box.
Remove fuse. Clean the fuse contacts and check the wire connections to the fuse box. Replace fuse.
Also check the power feed wire from control box terminal A.
If still a problem, then replace the fuse box and/or fuse.

Once the power circuit is verified, disconnect all test wires.
Reconnect the two striped harness wires to the horn.
Pressing the button should blow the horn.
If not, then grab the test light and re-trace everything listed above.
If you find something awry that I missed in these instructions, do tell.