The parking/running taillight is a great dust collector, mostly due to the huge openings top and bottom. Having a lot of dust stored in there (including in the grooves of the red lens) does not improve light levels!
Here is what they normally look like:
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I blocked mine off with foam in a friction fit. There are other ways to reduce the opportunity to get into the lens area such as Gorilla Tape or some form of plastic shield.
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If you do not have LED's installed, then you may want to maintain the ventilation. But even with the foam, there is still ventilation holes under the circuit board.
FYI