Buick Repair Questions? Ask a Mechanic for Answers ASAP
Welcome. Clean water leaking into the cabin is almost always because the drain is plugged. IF the other shops have tried to fix it I would hope this means they have checked the drain and cleared it. If this is the case and it has plugged again it leads me to thing there is more debris in the system that they are not clearing. I would be tempted to find the drain and use a shop vac to such as much out as you can. Likely the other shops used a wire or something to clear the drain.
It could also be that the rubber elbow is missing and the water is not draining past the sheet metal of the fire wall.
Sorry, the paste did not work. I will try again.
If the drain is clear then there is a crack or flaw in the heater case that is not allowing th water to drain out of the car. I have not seen this so it will take a sharp technician to find this flaw.