On vehicles in general the problem is almost always a crack or separation in the tail light. A tiny crack can admit a magical amount of water.

It is also common to find that the seal isn't working because of bad old bodywork. The mating surfaces may not mate up, or rust holes may have formed or they did not replace the factory caulking (expensive stuff).

I have found that trunks and lids rarely leak, except for similar bad bodywork issues. Same for quarter glasses.

The test is to use a hose just on the tail light, not above it or on the lid. If, the light is ok, then work up to a higher or different spot. This will isolate the area.

Sometimes the lens will fill up part way with water before it overflows into the quarter panel, or it will decide to not leak that day, so a give it time.

It is best to use a flashlight to watch the area inside while the water is applied. Even if you only see one drop, that is a leak area.

Almost all vehicles have drain plugs in areas where water can accumulate. I think it is nice of them, but it may also be part of the manufacturing process. Maybe for when they get dipped. Not sure.