This may not be the answer in your case, but certain older models of Buick (such as the Roadmaster) had a problem with the design of the air conditioning which sometimes caused the condensation to leak into the rear floorboards; after a while the carpet would start to mold and stink.
Water condensates on the cold condenser coils of the ac; just like water condensates on the outside of your cold drink on a hot day. If your ac does not drain this water properly it will leak.
HEATER CORE.
windshield leak that runs down behind the dash and onto the floor.
im a GM mechanic and i see these come through the dealership all the time. its eiter a severe coolant leak from the head gasket or your water pump has gone bad. both of these are serious conditions that you need to fix soon, before they cause engine damage
Leak on drain line above floor level
A water leak on the passenger side floor of a 2004 Saab 93 can be caused by several issues. Common culprits include a clogged or damaged sunroof drain, which can allow water to overflow and seep into the cabin. Additionally, a faulty door seal or a leak in the windshield seal can also contribute to water entering the vehicle. Lastly, a problem with the heater core, such as a leak, could cause coolant to accumulate in that area, mimicking a water leak.
rain water leak is caused by a bad windscreen seal
Probably, although it might be a heater hose. You will have to access the core in either case. If it smells like anti freeze then you may have a leak. If it is water then check your windshield seal. I have seen water leak down through the window post to the floor.
If the leak is by the passenger floor-- could be the heater core check to see if this leak is engine coolant or just water. Could be a leaking windshield seal could be a door seal could be evaporator line is plugged
if part of your vehicle was submerged or driven through a puddle, yes. The doors are not water tight and will allow water in from the bottom. Also water can get on the floor if your windshield leaks behind the dash board or if your heater has a leak in it.
Heater core leaking?
It could be the heater core leaking.