1988 Chevy s10 is leaking under dash when it rains

On a 1988 the most likely culprit is the rain tray under the wipers has rusted thru, or the base of the windsheild has rusted away at the bottom of the a pillars. Another possibility is an incorrectly installed replacement windsheild, or missing screws from around the wiper trim area, as the holes go into the interior of the truck and need screws in them to prevent leaks. The S10 trucks are notorious for rusting out.

Thanks for the suggestions. I will look to try and identify these areas. The rain tray(area where wiper arms are?) Appears solid. I will lopk for rusted bolts. The cowal is off and the base of the windshield appears to be okay. I dont know if this matters but The windshield was put in by a national company about three years ago and the technician made sure there was no rust or damage before he put it in because he explained that to me.

I will also look for the areas where the screws go into the frame.

Yep. The rain tray is the area underneath the wiper cowling, it is hidden by the trim around the wipers. This is also where the hvac system sucks in air, the compartment exists to allow water to flow into the tray, instead of being sucked into the cabin fan intake. Hence the 'rain tray' designation. There are drains at either end, but if these were clogged the water would most likely be pouring in through the hvac case under the dash, but it can happen. Do you park around a lot of pine trees? The needles can get in there and wreak havoc, but usually when that happens there is a strong moldy smell that comes thru the vents when the fan is on, as the constant wet and decomposing material would stink like crazy. You will have to remove the wipers and wiper cowling to see inside this area. If it is clogged, I suggest removing the front fender liners, as the tray drains dump the water behind them and they could be completely packed with crud.... Seen it before. I would be more suspicious of the windsheild in all honesty. Here's a test. Run a hose over the wiper cowling area only, make sure water comes out behind the front fenders, if it doesnt.... Start pulling parts and find out why.

I placed a hose in the area and the rain follows the rain tray down to the side. On the passenger side there is a flap that is stuck open. I will have to take off the quarter panel. The water is flowing there and i found some rusted out areas with holes under the dash and vent side. On the driver's side, the vent flap is stuck open and won't close so i will take that panel off also.

I checked the bottom of the windshield and water tray and both look solid