I got some condensation in my headlight after painting it. Here's what I did. I removed it (took off the bumper the first time, undid the top of the bumper and the Fender
screw the second time, pulled the edge out enough to get the ratchet in) ANYWAY, after I took it out, I used masking tape and followed the main seal all the way around. I put tape about 1mm from the seam on each side. Then I used Goop Automotive and put it in the seam. After I smeared it down with my finger I removed the tape. This left about a 2mm strip of glue all the way around. Haven't had any problems since.
If you ARE going to open your headlights to do this, take a bit of time and paint the insides. It's well worth the effort, especially if you plan on opening them anyway. Of course, I waited until the condensation dried before I re-sealed it.