Get a test light & start traceing the circuits. Like Tiny says one term on the stop light switch is hot all the time. When the pedel goes down the other term. should be hot. I don,t know about tump tump but if you get a clicking noise under the hood when you press the horn button it means the circuit is good up to the relay. Again use the test light to see vif any curent is getting to the horn. If not it,s a deffective relay. If it is it,s a bad horn.
