This problem has probably been there for a while. Once in a while, the day after it rained, I would take my canvas bag off the back floor and it would be wet. I always wrote it off to my having the back window not totally closed, but now I'm pretty sure it's something more.
Last weekend, I went to grab something from the back storage compartment, and everything it it was soaked as well. I thought my tailgate was rusting and leaking, and that may still be the case. I'm willing to bet that my door and tailgate seals are shot.
Let's see. In order to get my 86 Volvo back to good condition, I need new front struts, new rear shocks, new motor mounts, new transmission mounts, new transmission seals and gaskets, new headlight wiring of some kind, new turn signal wiring, new radio wiring, new rear defroster wiring (the existing wire is flat-out broken), new taillight lenses, a few new bulbs, new door and tailgate seals, a new jack, and at this rate, probably a new carpet, provided the floorboards haven't rusted through. All of this is on top of having four new tires in May ($400) and new brakes and rotors in June ($800).
I'm at the point that I'm ready to take the Volvo off the road and re-register and re-insure the minivan. It gets crappy mileage, the acceleration is for shit, there's body damage on both sides, the passenger mirror is broken, and it smells, but at least the headlights and radio work, and I'm not afraid of it breaking down on me.