scope103

Well, you need also to take into account a low battery. The horn pulls a lot more current than a test light, so it is possible to light up a test light with a very low battery, but the horn will just make a click sound.

Of course, if you follow RAD4Runner's advice (get a multimeter, for heck's sake!), the first thing you will do is measure the voltage at the battery terminals. If it's less than, oh, 12.2 volts, fix that first. (By charging the battery.)