I was having the same issue and picked up the replacement part from the dealership for $8. Thanks for the part number because the GM dealership had no idea what i was talking about and wanted to sell me the original rubber piece which cost more!
However... has anyone had issues with the drivers side getting wet? My carpet in the front drivers side is getting wet still and i ripped the carpet back and seen that its NOT coming in from the footwell area down the firewall by the pedals (it's also not coolant, its definitely water). So it must be originating from near the door somehow. I also checked the rubber plugs in the bottom of the floorboard and there still in nice and tight. Can anyone try and point me in the right direction on how to fix this?