December might not be the best time of the year to discover that there is water inside your car, especially when you live upstate New York... and your car is staying on an open air parking lot!
I waited for a snowless day, with temperature above freezing to venture taking a look at the problem. Water was accumulating under the front passenger seat, under the rug, threatening to turn the car into a mobile swamp. To find a leak, your only option is to spray every corner of the car with water and patiently observe and listen where the dripping comes from. It turned out that the water was coming from the trunk.
Under the rug in the trunk, both the spare wheel and the jack compartments were completely underwater. Driving the car was enough to jiggle the water under the back seat. Water would accumulate under the passerger front seat, due to gravity.
Step 1: Drying and testing
Since the water, was entirely on the passenger side, the leak had to be on that side too. I started by drying all the water inside the trunks. I had to take the rugs, the spare wheel and the jack out of the car. I needed something to scoop the the water out and had an empty bottle of shampoo with the top cut, slim enough to get the water stuck inside narrow compartments. I got as much water as I could like that and finished drying things with an old bath towel. Once everything is dry, it's much easier to see where water comes from!
The procedure is to spray the, beforehand closed (!!), trunk with the water from the pitcher. Get someone in the car, the back seat pulled down, with a flashlight to listen and watch where the water comes from (I managed to both spraying and spying). The picture shows what I saw: the water was coming from the backlight!