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The switch is in the rear brake line right under where the rear foot board bolts to the frame. is more than likely your problem . they fail all the time... simple test un plug both wires on it jump them together with a paper clip and see if your brake lite comes on.
My 07 Heritage did the same thing. Check your brake light switch. It runs along you frame rail. I believe there was a recall on the switch because there was so many problems.
99% probability it is the switch which is almost even with the right rear floorboard. Simple to swap out. My 08 Ultra rear brake switch went bad in 2 years. As others said, jump it or use a voltage meter to test. If I remember correctly it wasn't that expensive a part (for a HD part that is).
The equivalent switch from the autoparts store is about 9 bucks. Once you jump the wires and the light goes on, get out your metric wrenches, the switch is something like 13mm. Remember, you're introducing air into the system so you need to bleed the rear brake when you're done. Here's an optional way of doing it. https://www.hdforums.com/forum/sport...t-again-2.html