I am not familiar with this model but does it have a grill ahead of the windshield for air intake to the heater? If it does this probably leads to a tray that has drain holes in the corners. These drain holes may have rubber bungs with slits in them that can get blocked with leaves, the tray then fills up with water. When the car is driven the water swashes about and some goes into the heater intake and into the car. Remove the leaves and clean the bungs.
I am not familiar with this model but does it have a grill ahead of the windshield for air intake to the heater? If it does this probably leads to a tray that has drain holes in the corners. These drain holes may have rubber bungs with slits in them that can get blocked with leaves, the tray then fills up with water. When the car is driven the water swashes about and some goes into the heater intake and into the car. Remove the leaves and clean the bungs.
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Could be the heater core as Steve answered, but I would bet your HVAC housing drain is clogged. The heater /AC housing has a drain that terminates under the cab. When you run the AC, (and it can run while the heater is on as well), condensation is withdrawn from the air stream, collects in the bottom of the housing and drains out onto the ground. If the drain becomes clogged (a common problem) , the condensation backs up and flows out onto the floor of the cab. (Usually it will always drain on on side or the other due to the design of tbe housing).
Easy enough fix, involves crawling under the truck, identifying the drain opening, and sticking an air hose over the drain and blowing rhe clog open. Suppose you could use a can od dust blaster if you don't have an air compressor. Bugs are often found to have crawled in the hose ans died there.
Don't know the exact location of the drain on the Sierra, some vehicles terminate under the drivers side (close to the hump) and some terminate in the engine compartment.
One other point, if you find that this was the problem, often times the water that sat in the housing develops mold in the housing. (If you air vents smell funny, its probably mold). Best to treat this as its very unhealthy. I can recommed this foam to clean the HVAC housing and kill any mold or mildew. Just stick the hose up the end of the evaporator housing drain, push the button and discharge the contents inside the housing, it will turn to expanding foam, then back to liquid and drain out. Easy-peisy.
Lubegard 96030 Kool-It Evaporator and Heater Foam Cleaner https://www.amazon.com/dp/B0007PHD0S/ref=cm_sw_r_cp_apa_i_ak.NCbM1Y9104
SOURCE: Water in floorboard after every rain. 95 Dakota
the windshield could be leaking behind the dash,a good way to check is put the fan of the heater on,hot or cold and get a solution of soapy water and apply to the outer edges of the windshield and look for bubbles.
SOURCE: 2009 Ford Focus gets very wet floorboard in rear seat drivers side when driven in the rain
I used to get a shower when it rained, until I cleaned the sunroof drain holes (as Mike said).
Hope this helps.
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