Check the plenum, you will see on the passenger side there is a panel with a cut out ( on the Drivers side it is the same, but had your windscreen motor in it.) the rubber sael perishes and when rain water builds up in the try it rises enough to overflow into the cabin. Seal it around the edge with sikaflex.
Another point is any wiring that goes through the bulkhead with a grommet, and also the junction of the welds in the corner of the engine bay where the inner guard and bulk head meat, run some sikaflex across that also.

