2005 Chevy Impala a leak

Most likely some weatherstripping is not sealing properly, either windshield or door. You will have to find the leak. Put a dollar bill between door weatherstripping, close door, you should not be able to move bill, if you can, the door stripping is bad.

For windshield, use garden hose and spray one section at a time on windshield, then look for water inside, continue until you find leak area. To fix you will most likely have to remove the windshield and replace weatherstripping, or if you find a small area leaking, use a RTV sealant

Dry carpet with a fan, then apply a bleach solution, one cup to a gallon of water, it will kill mold. Then dry again.

Other possibility is floor pan sealant. Remove carpet and look for any sealant missing.