I agree with checking all the places noted in the previous posts. In my experience a leak around the windshield occurred at the bottom corners and you could detect water on the top of the dash way back in those corners.

Also check your rain gutters on the A pillars. I had 1 missing screw and that was all it took to do exactly what you describe. Water was able to run into that screw hole and down the pillar and right under the carpet of the front footwell, and then on back to the rear footwell.

__________________
1985 300D 231K anthracite grey w/black int.