Intermittent Working Horn

[spdway1]

85% of the time, my horn does not work. What fun is that?
It will work intermittently when I spin the horn button while pressing it. Do you think the ground under the horn button needs cleaning, or is a problem elsewhere?
Note: It has a brand-new wire harness, and the horn is strong when it works (lubricated properly and clean).

How do I go about fixing it?

(1930 Unrestored AA Grain Truck)

Thank you for your help and time.