While faulty parts are always suspect, don't overlook the possiblility of overloading your regulator. I imagine you have the 22Amp charging system. If you've added extra lighting, it can tax a charging system greatly...I know the previous owner of my electra glide added a cigarette lighter...this little piece of convienence pulls 6 amps by itself...enough to kill my second regulator...
If you want to add it up, here's a simple formula...Amps=Watts/Volts. Therefore, a 60W headlight draws 5 amps (60W/12V). I'm fairly sure my upgraded passing lamps are close to 60W each, so I'm running about 15 amps with just these three lights...let alone the taillight, front and rear running lights and radio...
Just something to keep in mind...you didn't mention adding anything like this, but I thought I'd throw it in... | 
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