How the &*^% did this happen????? Need gurus!!

[79BIG10]

The other day I was in my truck and the speaker on the rear driver side was not working. Well they're el cheapo 3 way enclosed speakers and the driver side was not working. I checked the connections and all was good. Then I unmounted the speaker and it weighed a ton!! IT WAS FULL OF WATER!!!!! It has been raining the other day but nothing else was wet in the cab. I took it out and tore it apart and it's shot to hell. Time for new better speaker but before that I want to fix this leak from somewhere. The seat, and everything else behind the seat is perfectly dry. No leaking from the rear window since I checked it with a water hose. I tried to find the leak but I can't find anywhere it would be. The speaker was mounted in the cab corner and nothing else was wet. Okay gurus WTF happened here??