2007 Chevrolet Equinox • 52,000 miles

Mystery? I noticed the floor of the back seat was soaked with water. I removed all the mats I had covering the floor. Water was in the middle and along the sides. No ceiling was wet or seats, no window was left open. Emptied the back of the hatchback, removed the floor pad (which was not wet) looked into the spare tire well and found standing water, about 3 inches in the deep part of the compartment. I don't know how long it has been there, and I presumed it has been sloshing around and absorbed into the carpet everytime I was driving. Question is - where is it coming from, or how is water getting in, what is leaking? All the plugs in the tire wells (outside) seem to be in place. Where do I begin to figure out how water is accumalating in the spare tire well? All the doors have drain holes. Any suggestions would be greatly appreciated.
November 3, 2012.

Windshield leak from fresh air vent allowing water in and settles in the rear area.