Start with obvious stuff first. You said it works in the park position, right? Sounds like something has to be right. Run a multimeter across the different positions on your ignition switch to make sure everything is going right and you have connectivity in the right places.
If nothing you can get a wiring diagram and check all of the electrical connections going to and from the taillight, but I don't think that will matter as you said the light works under certain conditions. If they are all connected, clean them all to ensure a good contact and reconnect. If still nothing, place a multimeter across each individual wire to check for connectivity.
Let us know the results, and we'll take it from there.
OH, and BTW it's a CB500K2. There is no "500F"
