Horn/light switch problem

[Jembow]

I took this '28 RPU apart about 12 yrs ago and I'm finally at the last stages of re-assembly.

Overhauled the Sparton horn, fooled around taking the horn rod out to cleanup the horn button, all works fine -AOOGAH! But when I turn the headlamp switch to main beam the horn goes off.

I have had a meter across the bottom rod terminal to earth and there is no leakage when I turn the light control, even took the switch apart and held the horn connection against the rod terminal - horn works but again no issues when I turn the light control. So logic says, it's not in the column, something is going on in the switch itself.

Any thoughts gratefully received.