First, check the bulb. If it's good, the problem commonly is worn or dirty contacts in the ignition switch, which has separate circuits for the taillight and main power. Verify with a VOM. Treat by disassembling the switch (watch out for the springs) and cleaning with contact cleaner. Polish the contacts with crocus cloth if corroded.
If a replacement switch is needed, you can get an alternative part from DK and probably other MO houses as well for much less than the Honda part. Disadvantage is that you would then need to carry two keys or have a new key made: one side cut for the gas cap/helmet lock and the other side cut for the ignition.