Why don't the brake light on 1983 Honda magna v45 motorcycle work?

Light circuits are very simple in operation but universally regarded a pig to trouble shoot; the wires go everywhere. The brakelight is operated by either the front or back brake levers; two switches. It is unlikely that both switches have failed simultaneously so check the grounding of the brake light first. Assuming the globe has been tested serviceable and the socket is OK, then look at the circuit fuse. Set ignition on and make sure there is battery voltage on both sides of the fuse. If no voltage to it, there is either a relay or ignition switch fault. Then check with ignition on there is battery voltage at the switches. To prove a switch, short out the bike side of the switch connector and the lamp should light. If still no brake light the fault is in the wiring loom and it is often more cost effective to run a new wire.