When asking tech questions it always helps if you include the model year and model name of the bike you're talking about.
Harley has had some issues with the rear brake light switch in recent years, and even had a recall at one point. Check to see if your bike was involved and if the recall was performed. See the attachment for details (click on it to make it full size).
If the brake light stays on, look for any obvious binding of the front brake lever or rear brake pedal that would prevent them from returning completely. The front lever can also fail to either contact the switch plunger, or press it in far enough, if the switch is loose inside the handlebar switch housing or the plunger has been damaged. Try removing one of the wires at the rear brake switch located in the brake line to see if that makes the light go out. If it does, the switch is either stuck or you have residual pressure in the brake line that isn't being released.
Jerry