2000 S: Horn Doesn't work when ignition 'On', works when 'Off'?

[rujorujo]

Horn works when ignition 'Off', doesn't work when ignition 'On'? I have wiring diagram and already confirmed 25A fuse B3 is good, and relay 10 is good. What can I try next (replacing ignition is out of the question to solve a horn problem)?

[newt]

I just went through this problem, exact same symptoms, horn works with no key, does not work when car is running. ..dealer replaced "clockspring" and airbag control unit in steering column....$675....ouch