Tell your friend that if he could tell that your tail lights weren't bright enough, then he was following too close.
God didn't make tail lights on old stovebolts bright. They were fine for the day.
Seriously, make sure all the connections are tight and the grounding point is purrrfect--no rust in the connections. All the voltage drop is supposed to be across the bulb filament, and when it ain't, then your 6v. bulb or whatever will glow at something less than 6 volts.
Awright! Enough out there about dim bulbs! And don't start no %$#&@ about Ohm's Law.
