[Joe K]

And, one thinks of the "ground" which while the car is "vibrating" probably makes a momentary ground good enough to sound the horn. While stationary maybe not so much.

Most horns have a single wire "feed" with the ground being made through the headlight bar. The usual issues with the headlight bar is "paint" - which gets in the way of both horn and headlight grounding. For the headlights the problem is worse simply because of more joints/paint layers.

But the horn may have its own affectation separate from the lighting.

Joe K