Water in 12V Battery Well

So I'm scratching my head over this one. My 12V battery just failed and when I replaced it I found water in the bottom of the well. My Prius lives outdoors, had snow on the roof. My first thought was that the water was getting in at the tail light, but on further examination I could actually see and feel water trickling vertically down the inside of the quarter panel straight into the well. The only entry points I can see as possibilities are the rear window or the body seam in the roof, but both would require the water to flow horizontally towards the rear of the car before going vertically into the well where I observed it. If the water got in at the roof seam I would envision it getting into the interior, unless there are additional internal panels that would divert it. Looking for ideas on this one!

 

Thanks very much for the lead. Sure sounds like mine. I'll be working on it today and let you know what I find. Fortunately, I think I caught it early. There was maybe 1/2" of water and it was very clear, and no rust at all. I popped out a rubber grommet in the floor of the well to let the water drain out.

 

Just finished checking for cracks and found a hairline crack just behind the clip for the body seam molding on BOTH sides. It's obvious that there is sufficient body flex to eventually crack the caulk and paint. I cleaned it thoroughly and put a bead of clear silicone over it. I do not think using paint would be a good fix as it is brittle. I checked the passenger side well too and found water there. I think this is an issue all owners of older Prius' should be aware of, as this is a long term maintenance issue. Thanks to all for the feedback that helped me identify this.

 

OK, I concur. I ordered some, but got it in a caulk tube. http://www.ebay.com/itm/Eastwood-Au...0434792363&pt=Motors_Automotive_Tools&vxp=mtr I'll post pictures when I strip off the silicone. Cheers!!