I just noticed gj sugested the same thing. WD40 is wonderful stuff but buy a big can and soak all electricals, switches, light sockets, any connectors, everything but the points, at least once a year.
I don't know why this would affect a car, but if your ped is wired like my Motobecane, the horn blows because you remove a dead short from one of the alternator windings when you push the button. This leaves the horn with 12 volts across it becaus it is connected all the time. So if my horn contacts corrode, the less they conduct, the louder the horn blows.
The button can also be turned to kill the engine. When I do that, it also unshorts the horn power and if I try to start, the horn blows at pedalling speed!