I recently purchased a 2013 Acadia Denali. There wes some condensation inside the rear tail lights at that time but I thought it was a temporary defect. I have had it apart and found someone had drilled 5 small drain holes in the assembly.

Further investigation showed that the condensation is inside the outer cover and the middle of the assembly. I took out the 6 screws but could not separate the two sections. They are likely ultrasonically welded. Sadly, an imperfect weld is allowing air inside which condenses during temp changes. I did note that while using a heat gun on the bulb hole (be very careful not to over heat) that the water ran out the bottom rear screwhole. About 1/2 teaspoon. After getting all the condensation out, I sealed up the 5 drilled holes which do NOT go into the affected area, and put it back together. If I still get condensation I will try leaving out the bottom rear inside screw so it can at least drain. No place for a little bag of desicant unless you place it near the circuit board which is likely also sealed.

I am not sure if this will fix the problem. Aftermarket lens is $230 C$

i have a line at eye level and very dim below that line

You cannot see at night any distance in front of you. OK in city but in country with no outside lighting it is horrible. You can only see five lines ahead of you on the road. Recommended by dealer to drive with fog lights on. WTF. Have complained to GMC as well as dealer, but I am the only one with complaint. Some guy made lots of money designing these headlamps. I will never buy another GMC. Too bad it cost so much money, otherwise vehicle would have been gone.