I posted this over in the Touring section, but I wanted to post it in here also.
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Let me apologize in-advance for a long winded post. I installed the Kuyakyn Bullet Light Rear Turn Signal Bar/Curved License Plate Frame on my 2003 FLHT. I'm not a very good shade-tree mechanic, but did it myself. When I had everything connected, I tested it before I bolted everything up. All lights were working perfectly. I also have rear spoiler lights on my saddlebags and the Harley Aux. Brake Light-Turn Signal adapter. When I bolted everything back up, I tested the lights again, and my right rear turn signal wasn't working. I unbolted the rear brake light and unplugged and replugged all the lights back in. Now, the right rear turn signal was working, but my rear brake light was not working, nor was brake light working on the turn signals or spoiler lights. Both rear turn signals work, running lights work on spoiler lights, fender light is working, and rear brake light running light is working, I just don't have any brake lights. A buddy with experience in electronics spent the week end using his voltage meter and the H-D service manual checking everything from the brake light relay to the rear brake light bulb. First thing he found was when I soldered the new plugs for the left and right turn signals, I didn't wrap them separate, I wrapped them together. He changed those around to how they should have been to start with, and then he checked everything. He also checked the system with the Aux. Brake/Turn Signal kit hooked up, and with it back to stock. Both was checked out OK. All voltages and plugs checked out OK. He was puzzled by 2 things:

(1.) With the brake light bulb unplugged from the rear fender assembly, I believe he was able to make the brake lights come on on the spoiler lights and turn signals by touching the red and black leads from his voltage meter to the brake light plug(red wire w/light colored stripe)and ground plug(black wire)in the rear fender assembly.

(2.) When he checked the voltage on the rear brake light relay, he got a reading of 6 volts in and 6 volts out. He wasn't sure if that was right or not.

Not knowing a lot about electronics, I hope I've explained this well enough you can understand.LOL Any help anyone can give will be greatly appreciated. Thanks in advance.

Hope everyone has a very Happy and Healthy New Year, B.J.

 

I experienced a similar problem installing a Signal Dynamics PentaStar module on my bike. My battery had discharged enough the voltage dropped a bit and the BRAKE function did not work. After hours of checking and rechecking connections I checked the brake light output - the meter read 11.28 volts. When I put the charger on the battery and the voltage went over 12 volts, the BRAKE function worked fine. The units are different but this might be worth a try.