Infiniti horn problem?

Yesterday I took my son for his drivers license road test and they would not allow me to do it because the horn would not work when the car engine is off. When the car is running the horn is fine, however when the engine is off no matter what position the key (in the ignition is) the horn will not work. by the way everything else (radio, lights, wipers) works just fine. It's almost like the horn has to warm up before it works....Anyone ever hard/seen this? suggestions to fix???