Or install a voltage regulator. One wire inline with the yellow to the lights and the other to ground. Once the volts go over 6V it will suck the extra to ground... I hear you can get one at radio shack for 1.50, not sure how true it is. I recently bought one from 1977 for 30 bucks... it does the trick though.
And yeah, no tail light is making it worse for you...