during a heavy rain, water will build up on the thresshold of the door.
Worse on the driver side.
I have had it back to the dealer twice.  They cleaned the drains and
they said it was water backing up inside the door and leaking out under
the upholstery.

BS!
The drains are clear and pretty dry.
The water is pooling to the *inside* of the door seal.  The seal is
doing its job and ..well...sealing. this prevents the water from
draining out.

24 hours after a good rain, I went out and opened the door and a couple
of ounces of water poured out onto the ground.  The other 4 cars, not
Mustangs, in the driveway did not do this.
Next day I drove to the dealership and when I pulled into the service
area and opened the door to get out, another couple of ounces poured
out.  So, it was dry, I drove it in dry weather, but the driving caused
more water to appear. To me that means it is pooling somewhere it is not
supposed to be and the movement caused more of it to land on the
thresshold.

The dealer said they pulled another new car off the lot and hosed it
down and it did just the same thing. So, there solution is ..."they all
do that."

I live in New England.  A pool of water on the thresshold, besides rust
issues, will freeze in the winter and if you can get the door open, you
may bend the hinges.

Anyone else experience this?

Norman