Mechanic stumped by water leak into driver's side floorboard.

I've read many posts regarding water in the floorboards, but none seem to solve my problem... any other suggestions???

2005 Camry LE (100k miles)
Hosing down the car could not replicate the leak, nor is it wet after a rain.
Cleaned out A/C, cowl panel, etc.  - no change.
Wheel wells do not appear to be cracked per Toyota's tech. bulletin.
I'm not sure whether the source is rain or A/C.  It has been both hot and rainy, so both are possible.  (There may be a better correlation to rain.)
Front carpets have been entirely removed.

The water is coming from UNDERNEATH the metal channel to which the front of the driver & passenger seats are bolted.  When I make a right turn (while decelerating) the water sloshes along the channel and forward into the floorboard.  The leak is from a small gap where the channel is welded to the floorboard, near to the trunk release.  Some water also reaches the passenger side floor (probably from left turns) but this is less common.  No water is apparent in the rear, though I wonder if the carpets are wet from underneath.

A single occurrence during a drive may result in only a couple of tablespoons of water and it's very rare for it to happen again the same trip.  A few days later, the "reservoir" has recharged and I have another leak the next time I make a decelerated turn.

Now that I know where the water is coming from, how is it getting in there???