Looks to me like the source was from a small puncture in the driver's footwell. Over time the wick effect drew moisture up. The beauty of the product I'm using is that it is completely unaffected by moisture.
Being that it was around 15F outside last night, I decided that the door panels needed to get finished. I reassembled them with 1/2" #14 screws and washers on the inside. Here's one on the door. Daddy like!
Got up early this morning and went to work. Got the rest of the paint and seam sealer stripped and sanded. Patches were made of sheet steel and coated front and back with POR15 prior to installation. Another great property of the POR15 product is its strength of adhesion. Once the area is dry I will verify they are fused in place. If there is any weakness I can apply some 3m panel bond, otherwise I'll be using 3m seam sealer just to make sure no moisture can come up through the areas.