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I have a 2009 Mercury Milan, premier package with sunroof etc.

I found a frozen spot of water (melted and smelled to confirm not antifreeze) about 1.75 inches in diameter near the center of the floor in front of my passenger seat. ("Near the center" = ~2" closer to the seat vs. the glove compartment and slightly left of center) The water had frozen due to the cold winter weather, so I could confirm that it did not expand much farther. I found the water before starting the vehicle.

  • Again, it's not wet near the glove compartment, firewall, door, or vertical frame. Wetness is isolated near the center of the carpet.

DETAILS: I had not driven this vehicle in over a week, and the car was parked facing downhill on a ~5% grade driveway. Weather in the last 4 days before I drove the vehicle: in the 50s (F) and raining for 2 days before dropping into the 20s (F; i.e., below freezing) for 2 days.

My thoughts:

  • Not AC or heater related since I had not driven the vehicle in 1+ weeks. Valid?
  • Not moisture coming from below car (e.g., due to rusted frame) since I had not driven in 1+ weeks
  • Likely due to rain 3 days prior to driving. Probably plugged up drain somewhere. But where?
    • Remember, it's parked on a 5% graded hill with the front of the car facing downhill. Could a leak at the windshield or cowl possibly fill up with water and drain "uphill"?
    • Or is it possible I have a sunroof drain plugged?
  • Or am I missing some other possibilities?

Not terribly hard to diagnose, I suppose, but I have no garage and tearing apart my car any more than I have to in subfreezing temps to find a possible leak is not something I want to drag out. If anyone has thoughts for me to narrow my options, I'd appreciate it!

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I can't say for sure about this vehicle but I've had the problem in the past where there is a small tube that connects to the sunroof drain (usually one in each corner) and this either deteriorates or works its way loose and allows the water to drain into the pillar instead of through the tube. Since the passenger footwell is the lowest point, I'd expect it to collect there.

Pull down the area of the headliner near the corner of the sunroof and inspect the drain tube.

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  • This was the direction I was leaning, but it's the drainage system I know least well -- adding to my hesitancy. Any procedural tips from your experience? (I'll research elsewhere, too, of course).
    – theforestecologist
    Dec 13, 2021 at 15:57
  • The third thing the drain can do is plug, which is probably the most likely, but has the same exact outcome. My wife's sunroof did exactly this not too long ago.
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Dec 13, 2021 at 19:31

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