Likely poor solder joint(s) in the LED bar itself.
The third brake light on the wifes 06 Vulcan failed a few years ago. Had power at the LED bar verified with a test light. Picked up a red LED light bar from Ebay for around $5 that was virtually the same length as the original. Removed the dead LED bar from the original housing and used JB Weld to glue the new LED light bar into the original housing. New LED light bar is somewhat brighter than the original.