Not sure if you are having trouble with the interior colored cap that goes over the assembly in your picture or the assembly you are showing taped down. In my case, the beige cap (that matches the rest of the plastic interior parts) would not stay in place. I used silicone (sparingly) to cement it in place. Haven't had to remove it to service the third brake light, but don't anticipate I'll have trouble removing it when I do.

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