water on backseat floor

i have no idea how water is getting under the carpet on the passengers side in the back seat. it is a 2002 Monte Carlo.

Does it only happen after a long trip with a/c on in humid weather? If so - check to see if your evaporator drain tube is attached-- seen this exact problem on B-bodies- water can back up into the air seals and roll out in behind carpet and back to backseat floorboard- the lowest point...

If no drain tube, then air pressure does not allow water to drain properly.