+1 for sunroof drain. Had something similar happen on my '07 after going through a touchless wash. Which was unusual, since I had been using touchless washes for years.
At first I thought I had a cracked heater core, but the moist part of the carpet didn't have a coolant smell and the coolant level was fine. You could see that the water had come in behind the dash on the passenger side. And I later recalled feeling a light mist of water from the roof when the spray arm passed over the top of the car during the last wash. In hindsight it all pointed to the sunroof drain holes.
Fortunately in my case the clog seemed to clear on its own. Honeslty I don't know of a test that doesn't involve more water, which would only add to your problem.
