In order for tail lights or head lights to not "fog up" by condensation, do I need to seal the fixtures tight? Or do I need to let them breathe.
Most fixtures seem to have a breathing tube, so I am assuming that I should let that be. But how do you keep them from fogging up?

It makes more sense to me to let them breath. That way you get equal pressure on the inside and outside of the light and the same air temp. Condensation or fog occurs when cool air hits hot air causing the moisture in the air to phase change or condense into a liquid. You can't create a vacuum around your lights so heat and cold are still going to pentrate so allowing same temp air to pass into lights allows less condensation.

I think I just confused myself. I say let'em breathe.

You want them to be sealed up. Not letting air in. Air carries moisture which once in the light after being outside in different temps from the weather it creates a fog look, condensation on the lens. You dont want that. Seal them completely.