I noticed some moisture in one of my tail lights today. Never had an issue until today. There has not been much rain as of late. However, I am washing my car more regularly with the weather finally cooperating. I took the lights out and here's what I found.
The driver's side (the one with the moisture) has 2 holes with something stuffed in it. Looks like sponge. Looks like someone modified it. No other holes where moisture or water can get in as far as I can see.
The passenger's side has black plugs with a rubber ring that seals tight. Inside these plugs is what looks like a small filter. Here's the question.. could that be desiccant? This light has no moisture. I realize that this is also (possibly) some sort of air filter.
I have no idea why someone would remove these plugs. Pics attached. I think in the interim I may stuff some desiccant from my wife's shoe box to see if that helps.

 

that would be a vent cap allows the housing to breathe. Light bulbs and LED's create heat so difference in temperature between inside and outside of the housing will create some condensation. i would check the seals on all of your bulbs in the tails to make sure they are in good condition. Or possibly could have put in a bulb wrong not making a complete seal with the housing.