There is really only two ways that water can get back there, the two being a leaky taillight gasket and the rear hatch gasket. Strangely enough, I would imagine the only way to get it all over the driver's seat would be the moonroof.

If you find a leak, fix it. even if it is only letting a few drops in a minute, remember, those few drops add up to a lot of water after eight hours of constant rain.

The rubber on the car is already eight to nine years old, so don't be suprised if it all could need replacement. It's not likely all three areas need it, but it won't hurt.