The Puch horn switch actually "shorts" the horn circuit to ground to cut the horn off. The horn spring-button inside the switch cap touches the horn wire lead when relaxed, and does not touch the horn wire lead when you press it. So, if the spring contact inside the switch cover is broken or missing, then the horn will continue to sound. I make a jumper wire to go between the horn wires, to complete the circuit.
As for your rear light, check the connectors going into the back of the tail light assembly, as lots of times these come unplugged. If that is not the problem, then check that you have power on the light switch. As for the brake light, check the connections on the brake light switches that are in the brake lever perches.