While you have it apart check the seal between the cowl and the heater . Pour water onto the cowl vent on the passenger side and see if any drains into the blower motor area. My truck came from Arizona and the foam seal deteriorated causing a leak. Rather than pull the dash and heater I just covered the air inlet cutout on the passenger side. I still get plenty of air from the cutouts on the drivers side and all the water goes down the left side now. No leaks.
