bertro is right. They are fastened from inside the shell.
Yes, the individual lights come out of the housing.
I found that when I removed the tail light lenses and the buckets that held the light fixtures I was able to reach inside the housing to retrieve the bullet connectors inside.
Most tail light problems aren't the wires themselves. Bad grounds, weak socket springs and corroded connectors are the main culprits.
That being said I still had a grounding issue with one housing. After cleaning the housing behind the the buckets I added new ground wires to the housings and ran them through to the inside behind the bath cabinets. Then down to the C channel on the floor.
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