Well I'm not sure which part has the tubing, the light itself (usually a short 90° piece) or the well the light assembly fits into. But they both would serve a purpose. Obviously the one for the well helps drain water from that area which is probably not watertight. Over time I guess dust and sirt got washed down into that tube and clogged it...they may have opted to not use a rubber seal around the light for any number of reasons. Odd that it got so full the wiring shorted, but great to hear you found the problem.
FYI the light assembly probably has one too, to help prevent condensation and fogging up of the lens by allowing the air in as well as allow pressure to equalize when the air inside gets hot from the bulbs. Newer LED versions may not need this since they run at such a low power.
Congrats on finding your problem and welcome to AF!