The horns, high and low are interlinked and have two electrical connections. Disconnect the wiring and using your meter check to see what's happening and are you sure both are working. If it sounds a bit weak then only one is functioning, if it blows your socks off then both are chirping. check those 4 electrical connectors and as Richard said the fuse. vibration from a running engine might be aiding a dodgy connection and the fuses and relays are notorious for slight corrosion. Whatever it is its going to be simple. As far as I remember there's now relay in the circuit.