Argh!! I'm not sure but some GM products are a nightmare to fix.

There is a little radiator in your heater unit under your dash on the passenger side. This unit has the hot radiator water circulated thru it to provide heat. After a certain year they figured out that if the water flowed through the heater core all the time it would last longer... It would also be hot all the time so when someone wanted heat it would appear "right now". It could also provide the needed hot water bypass for the engine at the same time.

Typically as VARA sez, it is a leaky core that is causing the problem.

Other possibilities are:

1) Leaky winshield.

2) Your air conditioner condensate drain is clogged or busted or cracked so the cold water that forms on the air conditioner evaporator coil is ending up in the car instead of draining out on the ground under the car.

3) A leak on the hose connection to the heater core that is running into your passenger compartment. (See the two hoses that leave the engine and head over to the dash.)

Troubleshooting.... Make sure the problem continues. Note COLD temperature radiator levels see if it is dropping. If you think the problem really is the core you can disconnect the two hoses at the dash and use a piece of pipe to connect them. You drive the car a few days and see if the water on the floor goes away. If it does.... It's the heater core! When your remove the hoses from the pipes coming thru the dash always use a sharp knife and just cut them off before the pipes. The pipes are VERY fragile and will break off, damage the core, or crush, if you try to get the hose off by wiggling, pulling, or prying. (The hose vulcanizes on to the pipe- self glues itself.)

Beware that people have been seriously scalded when heater cores rupture dumping BOILING water on the passengers feet.