Last Thursday I had the wifeperson in the car and it was raining the perverbal cats and dogs. I accelerated from a traffic light and the wife got a shower from under the dash. This was the first experience with this issue. As soon as we got home I pulled the mat and smelled the carpet (looking for a radiator smell, nothing just plain water.
I went on line to see if anybody else had experienced this problem, let me tell you hundreds had. I searched several TSBs and several Mustang forums, then the answer popped up. The proplem is with the drain hole located just to the center of the car by the A/C heater inlet. Turn on the ignition and turn on the windshield wipers when they get to the upright position shut the ignition off, then remove the right side (passinger) cover by removing the 3 plastic rivets, and gently lifting up to clear the upper catches move to the center and out.
You will see the filter to the air intake for the AC/heater, put your hand in and move it toward the center you will feel the drain plug, you will also see that if is full of junk. I used a long screwdriver to clear the drain, then some very MILD bleach water to kill any bacteria and flushed it with clear water several times.