High speed wiper not working on 1972 240z

I'm not an electrical expert but I believe acceleration should be increasing voltage in the car and so maybe the horn only works when you accelerate because at idle your voltage is too low. Have you tested voltage either across your battery or across the leads to the horn? I don't know what values they should be but it does sound like increase voltage is enough either through the relay or at the horn to make it work when you accelerate.