Yepper, same thing on mine. They work so I am not worrying. OTOH if someone has a fix, I'm paying attention!
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Yeah, prolly a gasket issue...mine both have moisture in them and have had it since I bought my Jeep 2 1/2 years ago. Hasn't damaged anything thus far so I just ignore it. You could probably silicone the assembly to the body, dry it out thoroughly and then silicone the cover shut to seal it. You'll have to redo it when you replace the bulbs...but that would solve it. Or just live with it...
"We don't build great brakes because we think you don't know how to drive, we build them for the guy on the road in front of you that doesn't." - Black Magic Brakes
I noticed moisture on one of my tail lights as well. I took off the tail light lense, put a very small beed of some flowable silicone glass sealer I had from another project. I think it was made by Permatex. I put it around the inside edge of the lense and let it dry (not sure how long it takes to dry because I let it dry overnight). It'll form a nice clear silicone type of gasket. In the mourning put the lense back on and haven't had any moisture since (about 8 months)