Ok to answer your question, to remove that vent box, there is two long bolts that have a "J" on the end that holds this in. On the bottom of the box is where the two bolts are. loosen the bolts and lift the 'j" hook over the lip in the cowl. Now where the water is coming from is the real question. Might be from the wiper pivots, cowl loaded with debris, or, rusted out cowl. Kind of common all of these. Or from around the windshield glass. This is a common issue with these cars. Car have carpet in it still? how are floors? rusted?