First inspect for cracks on both head and tail lights
If none are found its the lenses to back plastic seals themselves The RIGHT way to fix this is to set your oven to about 250*F and let them in there for like 10-15min then pull them apart and re seal.
Now there is another option, silicone. Pick up a big tube of black RTV silicone at home depot or lowes (the container that goes in the caulk gun). Now use that and seal all around the join where the lense meets the back plastic. Now for some extra insurance get some silica gel packets and stick them in the lights (if I remember right you cant get them in the rear lights, but the headlights have a area behind the projectors that arnt visible to the outside where a silica gel packet can sit.