why does water leak in to dash on chevy venture dash

I think it's your windshield not sealing properly, take it to a windshield place to have the gasket replaced, perhaps it's a door seal, would have to train a hose on the Chevy and wait and watch to see who's the culprit..I guess, before condemning the windshield water-tightness~

you're not gonna like this>>> http://www.carproblemzoo.com/chevrolet/venture/structure-problems.php

apparently common knowledge, problem 3 there, perhaps the Chevy dealer will have a heart knowing this problem~

There are seam seals and a water dam around the air intake in the cowl that sometimes dry out and leak. Remove the wiper arms and the cowl grill to take a look at them. Other common leaking areas are the door weather seals and sunroof drains.

Check for rust at the roof pinch weld where the glass seals.