I see. You have two bulbs in there. I also have aftermarket lights but they have single 6v dual filament bulbs.
I agree with Tiny on painting the inside of your taillights white. Most people think they should be silver/chrome but white is far superior. I even do that on 12v systems like the '66 Mustang. It's really amazing how much brighter the taillights appear when the inside is white.
I did that on the Packard which has very small taillights - about half the width of the one in Tiny's picture. I need all the light I can get on those things.
Otto, as crazy as a lot of people drive these days, I want to make sure I am visible when I drive my old truck!
Kevin