GM-GMC: Water Inside When It Rains, gmc sonoma, rain water

Question
1997 GMC Sonoma Ext Cab 4 x 4...The floor on the drivers side gets very wet when it rains. 2 dealerships have looked at it with no results. Yes the windshield has been replaced...not around door... I can't find where the water is coming from. It has been this way from the day I bought it. I do know the water is comeing in between back of engine...where heater blows out ect. Could there be some type plug missing on back of engine? HELP!

Answer
If it is clean water (rain water), someone will have to crawl up under the dash with a flashlight while water is sprayed on the vehicle.

If it is coolant, the heater core is leaking.

I do not know much about S-10/S-15 vehicles as I have never owned or worked on one of them.