Yes, the human is liable in that situation. The car manufacturer does not become liable just because the human did not press the horn. But if your car can only

Yes, the human is liable in that situation. The car manufacturer does not become liable just because the human did not press the horn. But if your car can only press the horn programmatically, and you never add the code to allow the horn to function, you don't actually have a horn, that's the vehicle's problem.