Not so sure about taking the headset off a150 super, big-ish job! Try testing all the wires to the horn for continuity while getting someone to rev the throttle,, (the engine should shouldn't to be running to do this at first, if you can't find the fault that way, the try with the engine running) especially look at any wire touching the throttle mechanism in the hane bars, I would remove the speedo or front light to see if any wires are moving! Good luck
Been thinking more about this. You may get the correct answer, quicker using the guys on the "modern vespa forum", the "not so modern" section, they are a bunch of good knowledgeable guys and I believe a few have/had 150 supers.
Plz remember to introduce yourself in the correct section first, it's only polite if your asking for help. Not a dig at you, just writing it for general consumption.