I had the same problem with my left tail light when I got it back from the Acura dealership during the 'snap ring' fix. It seemed that it had rained when it was outside (I'm surprised they left it out!DOAH!) and moisture got into the trunk area and seemed to evaporate straight into the tail lights. They told me the only way to fix it was to relace them (NOT!) Since I didn't want to spend the $$$, I got home and pulled out all the tail light bulbs and got a hairdryer and spent about 1/2 an hour blowing hot hair to dry the insides out. It worked! And from now on, when I wash the car, I open the trunk and specifically wipe around the drain areas between the weather stripping and the outside of the trunk, under the trunk lid and any other crevices that may retain water. I also do this after getting caught in the rain too. So far (knock on wood) the condensation has stayed out, without having to drill holes like others had done previously....
I hope this helps...
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Biaggi
92 Red NSX #483
94 CBR900RR